Musnad Abu Ya'la al-Mawsili 1:64

Ubayd Allah ibn Umar narrated to us, Affan ibn Muslim narrated to us, Hammam narrated to us, Thabit narrated to 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 beneath his feet, he would see us beneath his feet." He said: So he said: "O Abu Bakr, what is your thought regarding two, [of whom] Allah is the third of them?"