Sahih Muslim Book 31, Hadith Number 5868.

Anas b. Malik reported that Abu Bakr Siddiq reported him thus: I saw the feet of the polytheists very close to us as we were in the cave. I said: Allah’s Messenger, if one amongst them were to see at his feet he would have surely seen us. Thereupon he said: Abu Bakr, what can befall twain who have Allah as the third One with them.