Musnad Ahmad 27:443

'Abd al-Razzaq narrated to us, 'Ubayd Allah narrated to us, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) entered on the day of the conquest of Makkah upon a she-camel belonging to Usamah ibn Zayd, until he made it kneel in the courtyard of the Ka'bah. He called for 'Uthman ibn Talhah to bring the key, so he brought it and opened it. The Prophet (peace and blessings of Allah be upon him), Usamah, Bilal, and 'Uthman ibn Talhah entered. They closed the door upon themselves for a while, then they opened it. 'Abdullah said: I rushed ahead of the people and found Bilal standing at the door. I said: "Where did the Messenger of Allah (peace and blessings of Allah be upon him) pray?" He said: "Between the two front pillars." He said: And I forgot to ask him how many [rak'ahs] he prayed.