Musnad Ahmad 1013:22

Yunus ibn Muhammad narrated to us, Hammad narrated to us, Ubaydullah ibn Umar narrated to me, from Abu Hazim, from Sahl ibn Sa’d. Hammad said: Then I met Abu Hazim and he narrated it to me, and I did not deny anything of what he narrated to me. He said: There was fighting between the Banu ‘Amr ibn ‘Awf, and that reached the Prophet, may Allah bless him and grant him peace, after the Dhuhr prayer. So he came to them to reconcile between them and said to Bilal: "If the prayer time arrives and I have not come, then command Abu Bakr to lead the people in prayer." He said: When the prayer time arrived, he gave the Adhan, then he gave the Iqamah, and he commanded Abu Bakr, so he stepped forward. When he stepped forward, the Messenger of Allah, may Allah bless him and grant him peace, arrived. When he arrived, the people parted. He said: Abu Bakr used to not turn around when he entered into the prayer. He said: When he saw that they would not stop [parting], he turned around, and behold, it was the Messenger of Allah, may Allah bless him and grant him peace. He said: So he gestured to him with his hand to continue. He said: Abu Bakr retreated backwards. He said: And the Messenger of Allah, may Allah bless him and grant him peace, stepped forward. When the Messenger of Allah, may Allah bless him and grant him peace, finished the prayer, he said: "O Abu Bakr, what prevented you from continuing in your prayer when I gestured to you?" He said: He replied: "It is not for the son of Abu Quhafah to lead the Messenger of Allah, may Allah bless him and grant him peace, in prayer." Then he said: "If something happens to you during the prayer, let the men say 'SubhanAllah' and let the women clap."