Mishkat al-Masabih 691
‘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.)
وَعَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ دخل الْكَعْبَة وَأُسَامَة بن زيد وبلال وَعُثْمَان بن طَلْحَة الحَجبي فَأَغْلَقَهَا عَلَيْهِ وَمَكَثَ فِيهَا فَسَأَلْتُ بِلَالًا حِينَ خَرَجَ مَاذَا صَنَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: جعل عمودا عَن يَسَارِهِ وَعَمُودَيْنِ عَنْ يَمِينِهِ وَثَلَاثَةَ أَعْمِدَةٍ وَرَاءَهُ وَكَانَ الْبَيْتُ يَوْمَئِذٍ عَلَى سِتَّةِ أَعْمِدَةِ ثُمَّ صلى