Musannaf Ibn Abi Shayba 27:291

'Affan narrated to us, he said: Hammam narrated to us, he said: Thabit narrated to 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 we were in the cave: "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?"