Just clarification.
I am talking about the hadith about Salman Al-farisi (farisi = persian) رضى الله عنه not the hadith about Almohagirin and Alansar which is sahih as brother Mohsin mentioned.
Brother Ssalem was talking about hadith of Khurassan not of Slman Farsi(r.A)
.Abu Hurairah (R.A.) says that Rasulullah (Sallallahu Alayhi Wasallam) said: "(Armies carrying) black flags will come from Khorasan. No power will be able to stop them and they will finally reach Eela (Baitul Maqdas) where they will erect their flags.( Ref: Saheeh Trimdi)
13 .Hadhrat Buraidah (R.A.) says that Rasulullah (Sallallahu Alayhi Wasallam) said: "There will be many armies after me. You must join that army which will come from Khorasan.(Ref: Narrated by Ibn-e-Adi & Asaker & Seyuti in Jamiah Sagheer .Also in)
14. Narrated by Hazrat Abdullah bin Haris(R.A) that Rasulullah (Sallallahu Alayhi Wasallam) said that “People will come from the East who will make Khilafah (Islamic Rule) easy for Imam Mahdi.(Ref : Ibn-e-Majah Hadith 4088)
(This is also mentioned in Muslim)
I dont know what scholars say about these hadith .But which I am going to mention is definitely authentic.
"Thawbaan reported that the Prophet (Sallallaahu `alayhi wa sallam) said, 'Three will fight for the your treasure (of the Ka`bah), each of them the son of a 'Khaleefah', it will be rendered to none of them. Then from the direction
of the East will emerge black flags. Then they will fight you like they have fought none before.' Then some words were spoken which I did not remember. He then said, 'If you see him, give bay`ah to him even if you must crawl over ice. For, verily, he is the 'Khaleefah' of Allah (Khaleefatullaah), the Mahdi". This Hadeeth is found in Ibn Maajah, Ahmad's Musnad, al-Haakim's Mustadrak and others.
Now when analyzing the chain of narration found in Ibn Maajah's Sunan found the narrators to be the following:
1) Reported to Ibn Maajah by both Muhammad bin Yahya and Ahmad bin Yusuf - Both reliable; Imam Muslim has said about Ahmad bin Yusuf, "He is reliable (thiqqah)." an-Nasaa'i has said, "There is no problem in him." ad-DaraaquTni has said, "Reliable and noble (thiqqah nabeel)."
2) `Abdur-Razzaq bin Hammaam as-San`aani - Reliable. Ahmad bin Hanbal was asked about him, "Have you seen anyone better in Hadeeth than `Abdur-Razzaaq?" He replied, "No".
3) Sufyaan ath-Thawri - Too reputable to record his merits. Very reliable. Shu`bah, Sufyaan bin `Uyaynah, Abu `AaSim an-Nabeel and YaHya ibn Ma`een among others said: "Sufyaan is the 'Ameer al-Mu'mineen" of Hadeeth"!
4) Khaalid bin Mahraan al-Hathaa' - Reliable. Ahmad bin Hanbal has said, "Trustworthy (thabt)." Ibn Ma`een and an-Nasaa'i have both said, "Reliable (thiqqah)."
5) Abi Qulaaba Abdullaah bin Zayd al-Harrami - Reliable. Muhammad bin Sa`d has said in his "at-Tabaqah ath-Thaaniyah min Ahl al-Basrah" about him, "He was reliable (thiqqah)."
6) Abu Asmaa' `Amroo bin Marthad ar-RaHabi - Reliable. al-`Ijly has said about him, "Shaami, Tabi`ee, Reliable (thiqqah)."
7) Thawbaan - Companion and servant of the Prophet (Sallallaahu `alayhi wa sallam).
8) The Prophet (Sallallaahu `alayhi wa sallam). ["Tahtheeb al-Kamaal" of al-Mizzi and "Tahtheeb at-Tahtheeb" of Ibn Hajar were referenced for verification of the Narrators]
In this chain of narrators, weakness is nowhere in sight. Thus, focus has been taken away from the chain of narration and placed upon the term "Khaleefatullaah" for the Mahdi. They have said that Allah can't have a Khaleefah, since the word refers to a successor upon one's death, wa `eeyaathu billaah. Of course Allah is the Everlasting who can never die. Though they are correct that many of the scholars of objected linguistically (such as al-Maawardi in "aHkaam al-SulTaaniyah" while acknowledging a difference of opinion) remarking this is not a permissible term for the Pious Caliphs (citing Abu Bakr's and `Umar's objection to it), they have ignored the fact that the scholars of Tafseer have given this term legitimacy by exception. Imam al-Qurtubi states in his tafseer of the verse in which Allah's says about the creation of Adam (`alayhis-salaam), "Inny jaa`ilun fi-l arDee khaleefah.." ("verily, I will place in the earth a 'khaleefah'"): "And Adam, `alayhis-salaam, is the 'Khaleefatullaah' in the execution of his laws and orders. For verily he was the first of the Messengers. But there was none in the earth with him? It is said he was a Messenger to his progeny..." So hear Imam al-Qurtubi has found no fault in using the term Khaleefatullaah" in reference to Adam (`alayhis-salaam) about whom Allah said himself, "Inny jaa`ilun fi-l arDee khaleefah.." (al-Baqarah). In volume 6 of "FatH al-Baari SharH SaHeeH al-Bukhaari", Imaam ibn Hajar also uses the term in reference to the above mentioned verse about Adam (`alayhis-salaam) in the chapter, "Kitaab Ahaadeethi-l Ambiyaa', Baab Khalqi Aadam". So based on this Hadeeth's impeccable chain of narration, and no concensus forbidding the use of "Khaleefatullaah" specifically for the coming Mahdi, it is plausible to conclude that there may exist a particular bias against the subject of this Hadeeth by certain scholars from a certain part of the world. Those who have studied Hadeeth know that more leeway has been given for far weaker narrations with more questionable content.
Note.Khurassan is now a province of Iran.At the time of Prophet Khurassan was Afghanistan.Ancient Khurassan or Greater khurassan includes Afghanistan,Parts of Turkmenistan,Uzbekistan,Tajikistan,FATA,MALAKAND,NWFP,Punjab and parts of balochistan.