Sahih Muslim Book 007, Hadith Number 3073.
Chapter : The merit of entering the ka'ba for a pilgrim and observing of prayer in it.
Salim narrated on the authority of his father (Allah be pleased with him) that Allah's Messenger (may peace be upon him) entered the House along with Usama b. Zaid, Bilal and Uthman b. Talha. They closed the door from within, and, as they opened it, I was the first to get into it and meet Bilal, and I asked him: Did Allah's Messenger (may peace be upon him) observe prayer in it? He said: Yes, he observed prayer between these two Yemenite pillars (pillars situated towards the side of Yemen).