Musnad Ahmad 1312:2

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from 'Umarah, from Abu al-Sha'tha', who said: I went out for Hajj and arrived until I entered the House (the Ka'bah). When I was between the two pillars, I proceeded until I pressed against the wall. Then Ibn 'Umar came and prayed beside me, and he prayed four (rak'ahs). When he finished praying, I said to him: "Where did the Messenger of Allah, peace and blessings of Allah be upon him, pray in the House?" He said: "Usamah ibn Zayd informed me that he prayed right here." I said: "How many did he pray?" He said: "Regarding this, I find myself blaming myself; I stayed with him for a lifetime and did not ask him how many he prayed." Then I performed Hajj the following year, and I came and stood in his place, and Ibn al-Zubayr came and prayed in it four (rak'ahs).