Musannaf Ibn Abi Shayba 36:510

Hussain bin Ali narrated to us, from Za'idah, from Asim, from Zirr, from Abdullah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) was taken, the Ansar said, "An amir from us and an amir from you." He said: So 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 in prayer?" They said, "Yes." He said, "Then which of you is comfortable with himself to go ahead of Abu Bakr?" They said, "We seek refuge in Allah from going ahead of Abu Bakr."