The Prophet has said that two groups in my ummah (group of my followers) shall be successful. One which fights in Hind (India) and the other that accompanies Hadhrat Isa (Jesus) when he comes back.
Can we say that the freedom fighters of Kashmir belong to the first of the two groups mentioned by the Prophet (sws)?
The saying ascribed to the Prophet (pbuh), which you have alluded to in your question, is reported by Al-Nassaiy in his "Sunan" and by Ahmad ibn Hanbal in his "Musnad". It is reported in two different texts and through six different chains of narrators.
The first of the two texts is:
God shall save two groups of people from amongst my followers from hellfire. One, which shall fight in "Al-Hind" and the other, which shall accompany Isa ibn Maryam (Jesus) [on his return].
This text has been narrated, with very nominal differences, by the following two chains of narrators:
Mohammad ibn Abd Allah ibn Abd Al-Rahim --- Asad ibn Musa --- Baqiyyah --- Abu Bakr Al-Zubaidi --- Muhammad ibn Al-Walid Al-Zubaidi --- Luqman ibn `aamir --- Abd Al-a`laa ibn `adiy Al-bahraaniy --- Thauba'n --- The Prophet (pbuh). (As reported in Al-Nassaiy's "Sunan").
Abd Allah ibn Ahmad ibn Hanbal --- Ahmad ibn Hanbal --- Abu Nadhar --- Baqiyyah --- Abd Allah ibn Saalim --- Abu Bakr ibn Al-Walid Al-Zubaidi --- Muhammad ibn Al-Walid Al-Zubaidi --- Luqman ibn `aamir Al-Wassabi --- Abd Al-a`laa ibn `adiy Al-bahraaniy --- Thauban --- The Prophet (pbuh). (As reported in Ahmad ibn Hanbal's "Musnad")
The second of the two texts is:
The Prophet (pbuh) promised us the battle of "Al-Hind". Thus, if it happens in my lifetime, I should give away my life and all my belongings in it. If I am killed in it, I shall become the Best of martyrs (of Islam) and if I (live to) return, I shall become a "free" Abu Hurairah (i.e. free from Hellfire).
This text has been narrated by very nominal differences, by the following chains of narrators:
Ahmad ibn `uthman ibn Hakeem --- Zakariyya ibn `adiy --- `ubaid Allah ibn `amr --- Zaiyed ibn abi Unaisah --- Sayyaar --- Jabir ibn `abeedah --- Abu Hurairah. (As reported in Al-Nassaiy's "Sunan").
Ahmad ibn `uthman ibn Hakeem --- Zakariyya ibn `adiy --- Hushaim --- Sayyaar --- Jabir ibn `abeedah --- Abu Hurairah. (As reported in Al-Nassaiy's "Sunan").
Muhammad ibn Isma`eel ibn Ibraheem --- Yazeed --- Hushaim --- Sayyaar --- Jabir ibn `abeedah --- Abu Hurairah. (As reported in Al-Nassaiy's "Sunan").
Abd Allah ibn Ahmad ibn Hanbal --- Ahmad ibn Hanbal --- Hushaim --- Yasaar --- Jabir ibn `abeedah --- Abu Hurairah. (As reported in Ahmad ibn Hanbal's "Musnad")
Before we can derive any conclusions or making any applications, we must judge these narratives on at least the following two grounds:
The authenticity of the narratives, with reference to the reliability of its narrators; and
The basis of this narrative in either of the two primary sources of Islam, i.e. the Qur'an and the Sunnah or its basis in common sense.
In my opinion, the second criterion of judgment needs to applied only if the narrative seems to be acceptable on the basis of the first criterion of judgment (i.e. the reliability of its narrators). In view of this fact, I shall first of all present a brief analysis of the chain of narrators.
Narrators of The First Text
The First Set of Narrators:
1. Asad ibn Musa
The first chain of narrators, who have reported the first of the two texts includes "Asad ibn Musa".
The full name of Asad ibn Musa is "Asad ibn Musa ibn Ibraheem ibn Al-waleed ibn `abd Al-malik ibn Marwaan ibn Al-hakam". Ibn Hajar, in his book "Tehzeeb al-tehzeeb" has quoted sayings of Ibn Yunus, Ibn Hazam and `abd Al-Haq, besides a number of others' regarding Asad ibn Musa. Ibn Yunus says that Asad ibn Musa has narrated a number of abominable (Munkar) narratives. According to Ibn Hazam, Asad ibn Musa is abominable in his narratives and is a weak narrator. `abd Al-Haq says that he does not hold Asad ibn Musa's narratives as acceptable to be presented as a basis for arguments.
Asad ibn Musa has narrated this text from "Baqiyyah". The full name of Baqiyyah is: "Baqiyyah ibn Al-waleed ibn Al-Sa'id".
Al-Zahabiy, in his book "Meezaan al-Ai`tidaal" writes that more than one person has stated that Baqiyyah ascribes his narratives to such people from whom he has not heard these narratives. Al-Zahabiy has also quoted Ibn Habban as saying that Baqiyyah ascribes to reliable narrators such sayings which he heard from weak and unacceptable narrators. Al-Zahabiy has also cited similar comments (given below) of Abu Haatim, Abu Mus'har, Abu Is'haq Al-juzjaniy, Ibn Khuzaimah, Ahmad ibn Hanbal, `abd Al-Haq and Abu Al-Hasan ibn Al-Qattaan.
Abu Haatim says: "His (Baqiyyah's) narratives are not acceptable to be presented as a basis of an argument". Abu Mus'har gave his opinion about Baqiyyah in the form of a poetic verse, which means: "Narratives of Baqiyyah are not clean, so guard yourself against them". Abu Is'haq Al-juzjaniy says: "May God have mercy on Baqiyyah, he quoted worthless narratives without caring to check who was he taking such narratives from". Ibn Khuzaimah says: I do not hold Baqiyyah's narratives as acceptable to be presented as a basis of an argument. Ahmad ibn Hanbal is narrated to have said: "I thought that Baqiyyah only narrated acceptable narratives by ascribing them to unknown people. But then I found that he also narrates unacceptable narratives by (wrongly) ascribing them to known and reliable narrators". `abd Al-Haq says: "Baiyyah's narratives cannot be presented as a basis of an argument". Abu Al-Hasan ibn Al-qattan says: "Baiyyah (wrongly) ascribed his narratives to people and did not see any harm in doing so. If this is correct, it renders him unacceptable". Al-Zahabiy, commenting on this statement of Abu Al-Hasan writes: "By God, this is definitely correct".
Al-`uqailiy in his book "Dhu`afaa Al-`uqailiy" quotes Waqee` as saying: "I have not heard any person who was more daring than Baiyyah in ascribing something to the Prophet (pbuh). Al-`uqailiy has also quoted Ibn Al-Mubarak as saying: "He (Baqiyyah) would accept a saying from just about anyone [without checking his reliability]". Al-`uqailiy has also cited Ahmad ibn Hanbal as saying: "Baqiyyah did not care much about whom he was quoting from".
Al-hafiz Al-Mazi in his book "Tehzeeb Al-Kamaal fi Asma' Al-rijaal" has cited Yahya ibn Mu`een as saying: "He (Baqiyyah) would narrate a hundred narratives from weak and unacceptable narrators before he would quote one from a reliable narrator". He has also quoted Ya`qoob as saying: He (Baqiyyah) narrates from people whose narratives have been dropped and also from people who are weak and unacceptable in their narratives".
Ibn Hajar in his book "Tehzeeb al-Tehzeeb" has cited Abu Ahmad Al-Haakim as saying: He (Baqiyyah) has quoted narratives from Al-Awza`iy, Al-Zubaidiy and `ubaid Allah Al-`umariy narratives that seem to be lies". Ibn Hajar has also quoted Khateeb as saying: "Among his narratives are abominable ones". He has also quoted Al-Baihaqiy as saying: "There is a consensus regarding the fact that Baqiyyah is not fit to be presented as a basis of an argument".
3. Abu Bakr Al-Zubaidi:
The full name of Abu Bakr was "Abu Bakr ibn Al-Waleed ibn `aamir Al-Zubaidiy".
Not much could be found about Abu Bakr in books about the lives of people who have narrated sayings of the Prophet (pbuh). The little that was found is not very encouraging. Ibn Hajar in his book "Taqreeb Al-tehzeeb" writes: His life and character is not known".
Al-Hafiz Al-Mazi' in his book "Tehzi'b Al-Kamaal fi Asma' Al-Rijaal" and Ibn Hajar in his book "Tehzeeb Al-tehzeeb" write: Only Baqiyyah has narrated from him". This fact renders the narrator quite unreliable.
4. Muhammad ibn Al-Walid Al-Zubaidi:
Hafiz Ibn Hajar in his book Tehzeeb Al-tehzeeb has cited Al-Khalili as saying: "His (Mohammad ibn Al-Waleed Al-Zubaidiy's) narratives are reliable, if they are reported by a reliable narrator". This really means that the narratives of Muhammad ibn Al-Waleed have generally been accepted by the experts of the field. However, the case of the particular narrative under consideration is quite different. This narrative has been reported from Muhammad ibn Al-Waleed, by his brother, Abu Bakr ibn Al-Waleed, who is not even a known person. Thus the condition of acceptability of Muhammad ibn Al-Waleed's narrative, given by Al-Khalili (i.e. His narratives are reliable, if they are reported by a reliable narrator) is not fulfilled in this particular narrative of Muhammad ibn Al-Waleed.
5. Abd Al-A`laa ibn `adiy Al-Bahraaniy:
The comments regarding Abd Al-A`laa ibn `adiy Al-Bahraaniy although are generally quite positive, as he has generally been termed as a reliable and truthful person. But authorities like Ibn Al-qattaan have also said that "his (Abd Al-a`laa's) position as a narrator is not well known" (As reported in Ibn Hajar's "Tehzeeb Al-tehzeeb").
The Second Set of Narrators:
In the second set of narrators of the first text, we have some of the same weak, unacceptable and unreliable narrators as were present in the first chain of narrators, like Baqiyyah and Abu Bakr ibn Al-Waleed Al-Zubaidiy, and moreover, according to this chain, Abd Allah ibn Saalim has reported from Abu Bakr ibn Al-Waleed whereas, according to a number of authorities, no one other than Baqiyyah has reported from this person.
Narrators of The Second Text
The First Set of Narrators:
1. Zakariyya ibn `adiy
Although the comments regarding Zakriyya ibn `adiy are generally positive, but Al-Hafiz Al-Mazi in his book "Tehzeeb Al-Kamaal fi Asma' Al-Rijaal" and Hafiz Ibn Hajar in his book "Tehzeeb al-tehzeeb" have cited Ibn Nu`aim as saying: "What has he (Zakariyya ibn `adiy) got to do with the narratives [of the Prophet (pbuh)]. He was much more well versed with the Torah".
2. `ubaid God ibn `amr
The comments regarding `ubaid Allah ibn `amr are generally quite positive, except that Al-Zahabiy in his book "Tazkirah Al-Huffaz" and Ibn Hajar in his book "Tehzeeb Al-tehzeeb" have cited Mohammad ibn Sa`d as saying that although `ubaid Allah ibn `amr is generally quite reliable, yet sometimes he commits mistakes. Abu Wahab Al-Asadi has also been cited by Ibn Hajar in his book "Taqreeb Al-tehzeeb" as saying that he is sometimes prone to confusions.
3. Jabi'r ibn `abi'dah
The name of Jabi'r has also been reported as Jabr ibn `abeedah.
Al-Zahabiy in his book "Meezaan Al-ai`tidaal" writes that it is not known who Jabi'r was and his narrative regarding the Battle of Hind (India) - the narrative under consideration - is an abominable or a disagreeable one (munkar). Al-Hafiz Mazi in his book "Tehzeeb Al-Kamaal fi Asma' al-rijaal" writes that the narrative about the Battle of Hind is the only one narrated by Jabi'r.
The Second Set of Narrators:
The second set of narrators also includes the not very well known and thus not reliable, Jabi'r ibn `abi'dah. It also includes Hushaim.
His full name has been given as: Hushaim ibn Bushair ibn Al-Qasim ibn Di'naar Al-Sulamiy.
Ibn Hajar in his book "Tehzeeb Al-tehzeeb" has cited the following sayings about Hushaim: Al-`ujaliy says: that he is reliable but he ascribes his narratives to people from whom he did not hear those narratives. Ibn Sa`d says that he has narrated a lot of narratives but a lot of times he wrongly ascribed narratives to people from whom he had not heard them. Ahmad ibn Hanbal says that Hushaim has ascribed his narratives to a number of people whom he did not hear from. Incidentally, among these names, we also find Sayyaar, from whom Hushaim has cited the narrative about the battle of Hind. Ibn Al-Mubarak says that he asked Hushaim why does he wrongly ascribe narratives to people from whom he has not heard them, in reply Hushaim said: Your elders like A`mash and Sufian also did the same thing. This incident shows not only that Hushaim was guilty of wrongly ascribing narratives to people, but was also quite well known for doing so, even during his own times. Ibn Hibban has also given the same opinion about Hushaim as is expressed by those cited above. Ibn Sa`d has also said that unless Hushaim expressly says that "such and such person narrated to us" (which incidentally he has not done in the narratives under consideration except one in which he narrates from Sayyaar, who according to Ahmad ibn Hanbal, he did not hear from), his narratives are of absolutely no value.
The Third and Fourth Set of Narrators:
The third as well as the fourth set of narrators includes Hushaim, quoting from Sayyaar (there is no Yaasar among the people from whom Hushaim has narrated, thus there seems to be a spelling error in the fourth set of narrators due to which Sayyaar has become Yaasar. The two names in the Arabic language are written with the same alphabets.), whom he did not hear from, and it also includes the unknown Jabi'r ibn `abeedah.
In the preceding paragraphs, I have tried to briefly present an analysis of the narrators of the narratives ascribed to the Prophet (pbuh). As is quite obvious, not even one of these (six) chains consists of people who are reliable enough to allow us to say with any degree of confidence that the narratives under consideration are correctly ascribed to the Prophet (pbuh).
In view of this fact, there does not seem to be any need of going into further details regarding these narratives.
'Sunan' and 'Musnad' are the respective compilations of narratives ascribed to the Prophet (pbuh) of these scholars.
 i.e. to authenticate an actual saying of the Prophet (pbuh)
 i.e. to authenticate an actual saying of the Prophet (pbuh)