Sahih al-Bukhari 1201

It is related that Sahl said, "The Prophet, may Allah bless him and grant him peace, went out to make peace between the members of Banu 'Amr ibn 'Awf and the time of the prayer came. Bilal went to Abu Bakr and said, 'The Prophet, may Allah bless him and grant him peace, has been held up, so you should lead the people.' He said, 'Yes, if you wish.' Bilal called the iqama for the prayer and Abu Bakr came forward and started the prayer. Then the Prophet, may Allah bless him and grant him peace, came walking through the rows until he was standing in the first row and the people began to clap (tasfih)." Sahl asked, 'Do you know what tasfih is? It is the same as tasfiq.' "Abu Bakr was not someone who used to look round in his prayer but when the clapping increased he looked round and saw the Prophet, may Allah bless him and grant him peace, in the row. The Prophet indicated to him to stay where he was but Abu Bakr raised both his hands and praised Allah and stepped back and the Prophet, may Allah bless him and grant him peace, went forward and led the prayer."