From Sahih Muslim (Hadith 3236):
Salim b. Abdullah reported his father (Allah be pleased with him) saying:I saw Allah's Messenger (ﷺ) entering the Ka'ba, and Usama b. Zaid, Bilal and 'Uthman b. Talha were along with him, but none (else) entered therein along with them. Then the door was closed for them from within. 'Abdullah b. Umar (Allah be pleased with them) said: Bilal and Uthman b. Talha informed me that Allah's Messenger (ﷺ) observed prayer in the interior of the Ka'ba between the two Yemenite pillars
Reference: Muslim 3236