From Sahih Muslim (Hadith 3231):
Ibn Umar (Allah be pleased with them) reported:Allah's Messenger (ﷺ) came on the Day of Victory, and got down in the courtyard of the Ka'ba and he sent (a message) for 'Uthman b. Talha (Allah be pleased with them). He came with the key and opened the door. Allah's Apostle (ﷺ) then entered therein and Bilal, Usama b. Zaid, and 'Uthman b. Talha (along with him), and then commanded the door to be closed. They stayed there for a considerable time, and then the door was opened, and Abdullah said: I was the first to meet Allah's Messenger. (ﷺ). outside (the Ka'ba), and Bilal was close behind him. I said to Bilal: Did Allah's Messenger (ﷺ) observe prayer therein? He said: Yes. I said: Where? He said: Between the two pillars in front of his face. He said: I forgot to ask him as to the number of rakahs he prayed
Reference: Muslim 3231