مسند أحمد

It was narrated from Anas that Abu Bakr told him: I said to the Prophet (ﷺ) when he was in the cave - on one occasion he said: when we were in the cave -: If one of them looks at his feet, he will see us beneath his feet. He said: `O Abu Bakr, what do you think of two, of whom Allah is the third?`