Abu Sa'id al-Khudri (may Allah be pleased with him) relates that when the Prophet (peace and blessings be upon him) passed away, the orators of the Ansar stood up and said: O assembly of the Muhajirun! Whenever the Messenger of Allah (peace and blessings be upon him) sent one of you on a task, he would pair him with a man from among us. So our view is that this matter too should be taken on by two men, one from you and one from us. At that, Zayd bin Thabit (may Allah be pleased with him) stood up and said: The Messenger of Allah (peace and blessings be upon him) was from the Muhajirun, and we were the Ansar (helpers) of the Messenger of Allah (peace and blessings be upon him); so we are the helpers of whoever takes his (peace and blessings be upon him) place. Thereupon Abu Bakr (may Allah be pleased with him) said: "O assembly of the Ansar! May Allah reward you with the best reward as a tribe, and keep your speaker firm. By Allah! Had anything other than this been said, we would not have made peace with you."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4768