Sahih Bukhari, Chapter: Virtues of the Prayer Hall (Sutra of the Musalla)

'Abdullah bin 'Umar said, "Allah's Apostle entered the Ka'ba along with Usama bin Zaid, Bilal and 'Uthman bin Talha Al-Hajabi and closed the door and stayed there for some time. I asked Bilal when he came out, 'What did the Prophet do?' He replied, 'He offered prayer with one pillar to his left and one to his right and three behind.' In those days the Ka'ba was supported by six pillars." Malik said: "There were two pillars on his (the Prophet's) right side."