From Sahih Bukhari (Hadith 2988):
Narrated Nafi` from `Abdullah:Allah's Messenger (ﷺ) came to Mecca through its higher region on the day of the Conquest (of Mecca) riding his she-camel on which Usama was riding behind him. Bilal and `Uthman bin Talha, one of the servants of the Ka`ba, were also accompanying him till he made his camel kneel in the mosque and ordered the latter to bring the key of the Ka`ba. He opened the door of the Ka`ba and Allah's Messenger (ﷺ) entered in the company of Usama, Bilal and `Uthman, and stayed in it for a long period. When he came out, the people rushed to it, and `Abdullah bin `Umar was the first to enter it and found Bilal standing behind the door. He asked Bilal, "Where did the Prophet (ﷺ) offer his prayer?" He pointed to the place where he had offered his prayer. `Abdullah said, "I forgot to ask him how many rak`at he had performed
Reference: Bukhari 2988