Musannaf Ibn Abi Shayba 3:4788

Hussain bin Ali narrated to us, from Za'idah, from Asim, from Zirr, from Abdullah, who said: When the Prophet (peace and blessings of Allah be upon him) passed away, the Ansar said, "An amir from us and an amir from you." Umar came to them and said, "O assembly of the Ansar, do you not know that the Messenger of Allah (peace and blessings of Allah be upon him) commanded Abu Bakr to lead the people?" They said, "Yes." He said, "Then which of you is willing to step forward ahead of Abu Bakr?"