(52) باب الصَّلاَةِ فِي الْكَعْبَةِ
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: Sahih al-Bukhari 1599
In-book reference: Book 25, Hadith 85
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 26, Hadith 669