Sahih Muslim Book 7, Hadith Number 3069.
Chapter : The merit of entering the ka’ba for a pilgrim and observing of prayer in it.
Ibn Umar (Allah be pleased with them) reported: Allah’s Messenger (may peace be upon him) 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 (may peace be upon him) 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. (may peace be upon him). outside (the Ka’ba), and Bilal was close behind him. I said to Bilal: Did Allah’s Messenger (may peace be upon him) 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.