Musnad Ahmad 938:42

'Affan narrated to us, Wuhayb narrated to us, Dawud narrated to us, from Abu Nadra, from Abu Sa'id al-Khudri, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) passed away, the orators of the Ansar stood up. Some of them began to say: "O assembly of the Muhajirun, whenever the Messenger of Allah (peace and blessings of Allah be upon him) would appoint a man from among you, he would pair him with a man from among us. Therefore, we believe that this matter should be governed by two men: one from among you and the other from among us." He said: The orators of the Ansar continued to speak in that manner. He said: 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 (peace and blessings of Allah be upon him)." 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, had you done otherwise, we would not have reconciled with you."