From Sahih Bukhari (Hadith 1599):
Narrated Nafi`:Whenever Ibn `Umar entered the Ka`ba 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 Messenger (ﷺ) prayed, as Bilal had told him. There is no harm for any person to offer the prayer at any place inside the Ka`ba
Reference: Bukhari 1599