Sahih Muslim Book 007, Hadith Number 3074.
Chapter : The merit of entering the ka'ba for a pilgrim and observing of prayer in it.
Salim b. Abdullah reported his father (Allah be pleased with him) saying: I saw Allah's Messenger (may peace be upon him) entering the Ka'ba, and Usama b. Zaid, Bilal and 'Uthman b. Talha were along with him, but none (else) entered therein along with them. Then the door was closed for them from within. 'Abdullah b. Umar (Allah be pleased with them) said: Bilal and Uthman b. Talha informed me that Allah's Messenger (may peace be upon him) observed prayer in the interior of the Ka'ba between the two Yemenite pillars.