Musnad Ahmad 1013:21

'Affan narrated to us, Hammad bin Zayd narrated to us, Abu Hazim narrated to us, from Sahl bin Sa'd, who said: There was fighting between the Banu 'Amr bin 'Awf, and it reached the Prophet, peace and blessings of Allah be upon him. He came to them after the Dhuhr prayer to reconcile between them and said: "O 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 'Asr prayer time arrived, Bilal performed the Iqamah for the prayer, then he commanded Abu Bakr, and he stepped forward to lead them. The Messenger of Allah, peace and blessings of Allah be upon him, arrived after Abu Bakr had entered into the prayer. When they saw him, they clapped. The Messenger of Allah, peace and blessings of Allah be upon him, came, parting the people until he stood behind Abu Bakr. He said: Abu Bakr, when he entered into the prayer, would not turn around. When he saw that the clapping would not cease, he turned around and saw the Prophet, peace and blessings of Allah be upon him, behind him. The Messenger of Allah, peace and blessings of Allah be upon him, gestured to him with his hand to continue. Abu Bakr stood for a short while, praising Allah for that, then he walked backward. He said: The Messenger of Allah, peace and blessings of Allah be upon him, stepped forward and led the people in prayer. When the Messenger of Allah, peace and blessings of Allah be upon him, finished his prayer, he said: "O Abu Bakr, what prevented you from continuing when I gestured to you?" Abu Bakr said: "It is not for the son of Abu Quhafa to lead the Messenger of Allah, peace and blessings of Allah be upon him, in prayer." He then said to the people: "If something happens to you during your prayer, let the men say 'SubhanAllah' (tasbih), and let the women clap."