From Sahih Bukhari (Hadith 1598):
Narrated Salim that his father said:"Allah's Messenger (ﷺ), Usama bin Zaid, Bilal, and `Uthman bin abu Talha entered the Ka`ba and then closed its door. When they opened the door I was the first person to enter (the Ka`ba). I met Bilal and asked him, "Did Allah's Messenger (ﷺ) offer a prayer inside (the Ka`ba)?" Bilal replied in the affirmative and said, "(The Prophet (ﷺ) offered the prayer) in between the two right pillars
Reference: Bukhari 1598