From Sahih Bukhari (Hadith 5293):
Narrated Ibn `Abbas:Allah's Messenger (ﷺ) performed the Tawaf (around the Ka`ba while riding his camel, and every time he reached the corner (of the Black Stone) he pointed at it with his hand and said, "Allahu Akbar." (Zainab said: The Prophet (ﷺ) said, "An opening has been made in the wall of Gog and Magog like this and this," forming the number 90 (with his thumb and index finger)
Reference: Bukhari 5293