From Sahih Bukhari (Hadith 506):
Narrated Nafi':Whenever 'Abdullah entered the Ka'bah, he used to go ahead leaving the door of the Ka'bah behind him. He would proceed on till the remaining distance between him and the opposite wall was about three cubits. Then he would offer prayer there where the Prophet (ﷺ) had offered Salat, as Bilal informed me. Ibn 'Umar said, "It does not matter for any of us to offer prayers at any place inside the Ka'bah
Reference: Bukhari 506