The hadith narrated from Abu Hurayrah about the black banners is not saheeh
Question

                How sound is the hadith that was narrated by Abu Hurayrah (may Allah be pleased with him), according to which the Prophet (blessings and peace of Allah be upon him) said: “There will emerge from Khurasan black banners which nothing will repel until they are set up in Aelia”? Narrated by at-Tirmidhi. If this hadith is saheeh, where is Khurasan? May Allah reward you with good.
Praise be to Allah.
    The hadith mentioned in the question was narrated by the great Sahaabi Abu 
    Hurayrah (may Allah be pleased with him) from the Prophet (blessings and 
    peace of Allah be upon him) who said:

    “There will emerge from Khurasan black banners which nothing will repel 
    until they are set up in Aelia.” 

    Narrated by Nu‘aym ibn Hammaad in al-Fitan (1/213); Ahmad in al-Musnad 
    (14/383); at-Tirmidhi in as-Sunan (2269); at-Tabaraani in al-Mu‘jam 
    al-Awsat (4/31); al-Bayhaqi in Dalaa’il an-Nubuwwah (6/516); and 
    Ibn ‘Asaakir in Tareekh Dimashq (32/281) 

    All of them narrated it via Rishdeen ibn Sa‘d, from Yoonus, from Ibn Shihaab 
    az-Zuhri, from Qubaysah ibn Dhu’ayb, from Abu Hurayrah and attributed to the 
    Prophet (blessings and peace of Allah be upon him). 

    At-Tabaraani (may Allah have mercy on him) said: 
No 
    one narrated this hadith from az-Zuhri except Yoonus, and only Rishdeen 
    narrated it from him. End quote. 

    Al-Bayhaqi (may Allah have mercy on him) said: 
It 
    was narrated only by Rishdeen ibn Sa‘d from Yoonus ibn Yazeed. End quote. 

    This Rishdeen is the reason why this hadith is regarded as da‘eef (weak) and 
    munkar (odd). He was classed as da‘eef by Ahmad ibn Hanbal, and Yahya ibn 
    Ma‘een said concerning him: His hadith is not to be written down. Abu Zar‘ah 
    said: His hadith is da‘eef. Abu Haatim said: His hadith is munkar (odd), he 
    is negligent, he narrated odd reports from trustworthy narrators, and his 
    hadith is da‘eef. An-Nasaa’i said: His hadith is to be rejected. Abu Dawood 
    said: His hadith is da‘eef. Ibn Hibbaan said: His reports are mostly odd.

    See: Tahdheeb at-Tahdheeb (3/278) 
To 
    sum up: the hadith is da‘eef jiddan (very weak) and is not saheeh, because 
    Rishdeen ibn Sa‘d was the only one who narrated it; hence the scholars did 
    not accept it. 

    At-Tirmidhi said: It is ghareeb. End quote. This means that he regarded the 
    hadith as da‘eef. 

    Al-Bayhaqi said: 
A 
    similarly-worded report was narrated from Ka‘b al-Ahbaar. End quote. 
In 
    fact, when al-Haafiz Ibn Katheer narrated a number of hadiths that mentioned 
    the black banners, he said: All of this leads one to doubt the soundness of 
    these hadiths, as none of their isnaads is free of problems. 

    End quote from al-Bidaayah wa’n-Nihaayah (6/248). 

    The hadith was classed as da‘eef by al-Haafiz Ibn Hajar in al-Qawl al-Musaddad 
    (42); Shaykh al-Albaani in as-Silsilah ad-Da‘eefah (no. 4825); Shaykh 
    Ahmad Shaakir in his commentary on Musnad Ahmad (16/316). The 
    commentators on Musnad Ahmad (Mu’sasat ar-Risaalah edn.) said:

    Its isnaad is da‘eef jiddan (very weak). End quote. 
In 
    fact they also said: 

    There is no saheeh report about this matter; there are only da‘eef (weak) 
    and mawqoof (i.e., the isnaad does not go back to the Prophet (blessings and 
    peace of Allah be upon him)) reports. End quote. 

    Similarly, Shaykh Dr Haatim ash-Shareef said: 

    There is no saheeh, marfoo‘ hadith about the black banners, and there is no 
    hadith that has an isnaad going back to the Sahaabah (may Allah be pleased 
    with them). 

    End quote from a study on hadith published for the first time on the Islam 
    today website, then published in Multaqa Ahl al-Hadith

    Anyone wanting to know more detail on the evaluation of the hadiths that 
    speak of this topic may refer to the study mentioned above. 

    Whatever the case, al-Haafiz Ibn Katheer (may Allah be pleased with him) 
    said that, even if we assume that the hadith is sound, 

    These black banners are not those with which Abu Muslim al-Khurasaani came 
    and overthrew the Umayyad state in 132 AH; rather it refers to other black 
    banners with which the Mahdi will come. End quote from al-Bidaayah 
    wa’n-Nihaayah (19/62) 
It 
    should also be noted that Khurasan is a large territory that includes the 
    north-east of Iran; it extends from Jarjaan and Tabaristan on one side, to 
    Transoxiana on the other side. From a political point of view it includes 
    Transoxiana and Sijistan (now Afghanistan). The region that is now known as 
    Khurasan encompasses less than half of historical Khurasan; the rest of that 
    region now belongs to Afghanistan.

    End quote from Ta‘reef bi’l-A‘laam al-Waaridah fi’l-Bidaayah 
    wa’n-Nihaayah li Ibn Katheer (1/471) 

    And Allah knows best.