Musnad Ahmad 694:1

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Ibn Shihab al-Zuhri said: 'Abd al-Malik ibn Abi Bakr ibn 'Abd al-Rahman ibn al-Harith ibn Hisham narrated to me, from his father, from 'Abd Allah ibn Zam'ah ibn al-Aswad ibn al-Muttalib ibn Asad, who said: When the Messenger of Allah (may Allah bless him and grant him peace) became severely ill, and I was with him among a group of Muslims, he said: Bilal called for the prayer, so he said: "Command someone to lead the people in prayer." He said: So I went out, and behold, 'Umar was among the people, and Abu Bakr was absent. So I said: "Stand, O 'Umar, and lead the people in prayer." He said: So he stood, and when 'Umar said the takbir, the Messenger of Allah (may Allah bless him and grant him peace) heard his voice, and 'Umar was a man who projected his voice. He said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Where is Abu Bakr? Allah refuses that, and the Muslims refuse that. Allah refuses that, and the Muslims refuse that." He said: So he sent for Abu Bakr, and he came after 'Umar had finished that prayer, and he led the people in prayer. He said: 'Abd Allah ibn Zam'ah said: 'Umar said to me: "Woe to you, what have you done to me, O Ibn Zam'ah? By Allah, I did not think when you commanded me [to lead] except that the Messenger of Allah (may Allah bless him and grant him peace) had commanded you to do so, and had it not been for that, I would not have led the people in prayer." He said: I said: "By Allah, the Messenger of Allah (may Allah bless him and grant him peace) did not command me, but when I did not see Abu Bakr, I saw you as the most worthy of those present to lead the prayer."