Sahih Bukhari, Chapter: Pilgrimage (Hajj)

Whenever Ibn 'Umar entered the Kaaba he used to walk straight keeping the door at his back on entering, and used to proceed on till about three cubits from the wall in front of him, and then he would offer the prayer there aiming at the place where Allah's Apostle prayed, as Bilal had told him. There is no harm for any person to offer the prayer at any place inside the Kaaba.