Sahih Muslim Book 007, Hadith Number 3071.
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) entered the House, and Usama, Bilal and Uthman b. Talha were with him, and they kept the door closed for a considerable time. Then it was opened and I was the first to enter the House and meet Bilal, and I said: Where did Allah's Messenger (may peace be upon him) observe prayer? He said: Between these two front pillars. I, however, forgot to ask him the number of rak'ahs that he observed.