Sahih Bukhari, Chapter: Prayers (Salat)

Someone came to Ibn 'Umar and said, "Here is Allah's Apostle entering the Ka'ba." Ibn 'Umar said, "I went there but the Prophet had come out of the Ka'ba and I found Bilal standing between its two doors. I asked Bilal, 'Did the Prophet pray in the Ka'ba?' Bilal replied, 'Yes, he prayed two Rakat between the two pillars which are to your left on entering the Ka'ba. Then Allah's Apostle came out and offered a two-Rak'at prayer facing the Ka'ba.' "