Musannaf Ibn Abi Shayba 36:79

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, may Allah bless him and grant him peace, 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 thought regarding two, the third of whom is Allah?"