Musannaf Ibn Abi Shayba 36:506

'Affan narrated to us, Wahib narrated to us, Dawud narrated to us, from Abu Nadra, from Abu Sa'id, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) passed away, the orators of the Ansar stood up. One of them began to say: "O assembly of the Muhajirun, the Messenger of Allah (peace and blessings of Allah be upon him) used to, when he appointed a man from among you, pair him with a man from among us. So we believe that this matter should be governed by two men, one of them from you and the other from us." He said: The orators of the Ansar followed one another in that. Then Zayd ibn Thabit stood up and said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) was from the Muhajirun, and the Imam must be from the Muhajirun, and we are his helpers (Ansar) just as we were the helpers of the Messenger of Allah." Then Abu Bakr stood up and said: "May Allah reward you with good, O assembly of the Ansar, and may He make your speaker firm." Then he said: "By Allah, if you had done other than that, I would not have reconciled with you."