Musannaf Ibn Abi Shaybah 39834 — Reports on the caliphate of Abu Bakr

Muhammad (bin Sirin) says that at the time of the pledge of allegiance to Sayyiduna Abu Bakr (may Allah be pleased with him), the people went to Sayyiduna Abu Ubaydah bin al-Jarrah (may Allah be pleased with him), and he said: "You come to me, while among you is the third of three?" meaning Sayyiduna Abu Bakr. Ibn Awn says: I asked Muhammad: "Who is the third of three?" He replied: "The saying of Allah Almighty: 'the second of two when they were in the cave' [Al-Tawbah: 40]."