Sahih Bukhari Volume 002, Book 022, Hadith Number 293.
Narated By Sahl bin Sad : The Prophet went out to affect a reconciliation between the tribes of Bani 'Amr bin 'Auf and the time of the prayer became due; Bilal went to Abu Bakr and said, "The Prophet is detained. Will you lead the people in the prayer?" Abu Bakr replied, "Yes, if you wish." So Bilal pronounced the Iqama and Abu Bakr led the prayer. In the meantime the Prophet came crossing the rows (of the praying people) till he stood in the first row and the people started clapping. Abu Bakr never looked hither and thither during the prayer but when the people clapped too much, he looked back and saw the Prophet in the (first) row. The Prophet waved him to remain at his place, but Abu Bakr raised both his hands and sent praises to Allah and then retreated and the Prophet went forward and led the prayer. (See Hadith No. 295 & 296)