Sahih Bukhari Volume 7, Book 63, Hadith Number 215.

Narrated By Ibn Abbas : Allah’s Apostle 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 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).