Sahih Bukhari
Virtues of the Prayer Hall (Sutra of the Musalla)
Volume: 001 / Book Number: 009 / Hadith Number: 483
Narrated Ibn 'Umar:
The Prophet entered the Ka'ba along with Usama bin Zaid, 'Uthman bin Talha and Bilal and remained there for a long time. When they came out, I was the first man to enter the Ka'ba. I asked Bilal "Where did the Prophet pray?" Bilal replied, "Between the two front Pillars."