Musnad Ahmad 27:1152

He said: And he informed me that Abdullah ibn Umar informed him, "That the Messenger of Allah, peace and blessings of Allah be upon him, faced the two gaps of the long mountain that is before the Kaaba. He placed the mosque that was built to the right, and the mosque at the edge of the hill, and the prayer place of the Messenger of Allah, peace and blessings of Allah be upon him, below it, on the black hill. He would leave ten cubits or thereabouts from the hill, then he would pray, facing the two gaps of the long mountain that is between him and the Kaaba."