Musnad Ahmad 27:727

Yahya narrated to us, from Ubaydullah, Nafi’ narrated to me, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) entered the House [the Kaaba], along with Bilal, Usama ibn Zayd, and Uthman ibn Talha. They closed the door behind them and remained for a while. Then he came out. When it was opened, I was the first to enter. I asked Bilal: "Where did the Messenger of Allah (peace and blessings of Allah be upon him) pray?" He said: "Between the two front pillars." And I forgot to ask him how many [rak'ahs] he prayed.