Sahih Bukhari
Sahih Bukhari Volume 005, Book 057, Hadith Number 005.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 57. Companions of the Prophets
Narated By Abu Bakr : I said to the Prophet while I was in the Cave. "If any of them should look under his feet, he would see us." He said, "O Abu Bakr! What do you think of two (persons) the third of whom is Allah?"