Mishkat al-Masabih #643

‘Abdallah b. ‘Umar said: God's Messenger entered the Ka'ba with Usama b. Zaid, ‘Uthman b. Talha al-Hajabi, and Bilal b. Rabah, and locking it behind him, he remained inside. I asked Bilal when he came out what God's Messenger had done, and he said, “He took up a position with one pillar on his left, two on his right, and three behind him (the House at that time having six pillars), then performed salat.” (Bukhari and Muslim.)