Musnad Abu Ya'la al-Mawsili 1:65

Abu Khaythama narrated to us, Habban ibn Hilal narrated to us, Hammam narrated to us, from Thabit, who said: Anas narrated to us that Abu Bakr narrated to him, saying: I looked at the feet of the polytheists above our heads while we were in the cave, so I said: "O Messenger of Allah, if one of them were to look at his feet, he would see us beneath his feet." He said: "O Abu Bakr, what is your opinion of two, the third of whom is Allah?"