From Sahih Muslim (Hadith 6169):
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 two who have Allah as the third One with them
Reference: Muslim 6169