God said to Moses, "This is the land of which I swore to Abraham, to Isaac, and to Jacob, 'I will give it to your offspring'" (Deuteronomy 34:4).
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) 
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)