From Sahih Bukhari (Hadith 504):
Narrated Ibn `Umar:The Prophet (ﷺ) entered the Ka`ba along with Usama bin Zaid, `Uthman bin Talha and Bilal and remained there for a long time. When they came out, I was the first man to enter the Ka`ba. I asked Bilal "Where did the Prophet (ﷺ) pray?" Bilal replied, "Between the two front Pillars
Reference: Bukhari 504