Black Banners from the East
The following narrations state that an army from the East (most likely from Khorasan) carrying black banners will lay the foundation for the Mahdi's future dominion, six years before the Mahdi appears. The name "Khorasan" is said to be derived from Persian words that mean "land where the sun rises". Khorasan (a region that includes Afghanistan, Taijikstan, Northwestern Pakistan, Northeastern Iran, etc.).
Abdullah bin Masood narrated that the Prophet صلى الله عليه وسلم said : "A group of people will come from the East with Black Flags and they will ask for some “Khair” (financial assistance, share in wealth, weapons, etc.), but they will not be given what they ask for. So, they will fight and win against those people (who didn’t give them what they asked). Now, they will be given them what they asked for, but they will not accept it until they hand it over to a person from my Ahl Bayt (family) who will fill it (the Earth) with justice just as they filled it (the Earth) with oppression. So, whoever of you will be alive at that time should come to them (the Mahdi & People from the East with black flags) even by crawling over snow". (Ibn Maja, Muhammad Al-Barzanji's Isha'ah li Ashrat Al-Sa'a)
Abdullah bin Al-Harith bin Jiz Al-Zubaidi said that the Prophet صلى الله عليه وسلم said : "People from the East will come to assist the Mahdi in laying the foundation for his dominion." (Ibn Maja, Tabarani)
Abu Hurairah says that the Messenger of Allah صلى الله عليه وسلم said: "(Armies carrying) black flags will come from Khorasan . No power will be able to stop them and they will finally reach إيلياء Eelya (Jerusalem) where they will erect their flags". (Tirmizi)
Amr Bin Marra Al-Jamli said that the Prophet صلى الله عليه وسلم said: " 'Surely, Black Flags will appear from the Khorasan until a group (of them) will tie their horse leashes to the Olive Trees between Beit-Lahia (in Gaza district, Palestine) and Harasta (near Damascus, Syria).' We asked: 'Are there any Olive trees between these places?' He said, 'If there isn’t any, then, soon (Olive trees) will grow so that those people (of Khorasan) will come and tie their horses there'.” (Nuaim Ibn Hammad's Kitab Al-Fitan p. 215)
Buraida said that Messenger of Allah صلى الله عليه وسلم said: "After me, there will be many expeditions (armies). You should be in (or with) the expeditions (armies) of (coming from) Khorasan." ( Jalal-uddine AsSuyuti's Al-Urf Al-Wardi fi Akhbar Al-Mahdi, a part of Al-Hawi li Al-Fatawa)