Musnad Ahmad 1:11

'Affan narrated to us, he said: Hammam narrated to us, he said: Thabit informed us, from Anas, that Abu Bakr narrated to him, he said: I said to the Prophet, peace and blessings of Allah be upon him, while he was in the cave—and he said once: "And we were in the cave"—: "If one of them were to look at his feet, he would see us beneath his feet." He said: So he said: "O Abu Bakr, what is your opinion of two, the third of whom is Allah?"