Sahih Bukhari Volume 002, Book 026, Hadith Number 668.
Narated By Salim : That his father said, "Allah's Apostle, Usama bin Zaid, Bilal, and 'Uthman bin abu Talha entered the Ka'ba and then closed its door. When they opened the door I was the first person to enter (the Ka'ba). I met Bilal and asked him, "Did Allah's Apostle offer a prayer inside (the Ka'ba)?" Bilal replied in the affirmative and said, "(The Prophet offered the prayer) in between the two right pillars."