Musnad Ahmad 26:218

Husayn ibn 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) passed away, 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? So which of you would be comfortable with himself to step forward ahead of Abu Bakr?" They said: "We seek refuge in Allah from stepping forward ahead of Abu Bakr."