Musannaf Ibn Abi Shayba 36:517

Abu Usama narrated to us, from Ibn 'Awn, from Muhammad, from a man from Banu Zurayq, who said: When that day came, Abu Bakr and Umar went out until they came to the Ansar. Abu Bakr said: "O assembly of the Ansar, we do not deny your right, and no believer denies your right. And by Allah, we have not attained any good except that you have shared it with us. However, the Arabs will not be pleased nor will they settle except under a man from Quraysh, because they are the most eloquent of people in tongue, the best of people in countenance, the most central of the Arabs in abode, and the most numerous of people in nature among the Arabs. So come to Umar and pledge allegiance to him." He said: They said, "No." Umar said, "Why?" They said, "We fear favoritism." Umar said, "As for as long as I live, no." Then they pledged allegiance to Abu Bakr. Abu Bakr said to Umar, "You are stronger than me." Umar said, "You are better than me." They said it a second time. When it was the third time, Umar said to him, "My strength is for you, along with your excellence." He said: So they pledged allegiance to Abu Bakr. Muhammad said: And when the people came at the time of the pledge of allegiance to Abu Bakr, they came to Abu Ubaydah ibn al-Jarrah. He said, "Do you come to me while among you is the second of two?" meaning Abu Bakr. Ibn 'Awn said: I said to Muhammad, "Who is the second of two?" He said: The saying of Allah: "The second of two, when they were in the cave."