Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2892
Nafi' reported that Abdullah (b. 'Umar) informed him that Allah's Messenger (may peace be upon him) turned his face to the two hillocks which intervened between him and the long mountain by the side of the Ka'ba, and the mosque which had been built there was thus on the left of the hillock. Allah's Messenger's (may peace be upon him) place of prayer was lower than the black hillock, at a distance of ten cubits or near it. He (may peace be upon him) would then observe prayer facing these two hillocks of the long mountain that is intervening between you and the Ka'ba.