Sahih Bukhari Volume 5, Book 58, Hadith Number 259.

Narated By Abu Bakr : I was with the Prophet in the Cave. When I raised my head, I saw the feet of the people. I said, “O Allah’s Apostle! If some of them should look down, they will see us.” The Prophet said, “O Abu Bakr, be quiet! (For we are) two and Allah is the Third of us.”