Sahih Bukhari Volume 9, Book 89, Hadith Number 300.

Narrated By Sahl bin Sa’d As-Saidi : There was some quarrel (sighting) among Bani ‘Amr, and when this news reached the Prophet, he offered the Zuhr prayer and went to establish peace among them. In the meantime the time of ‘Asr prayer was due, Bilal pronounced the Adhan and then the Iqama for the prayer and requested Abu Bakr (to lead the prayer) and Abu Bakr went forward. The Prophet arrived while Abu Bakr was still praying. He entered the rows of praying people till he stood behind Abu Bakr in the (first) row. The people started clapping, and it was the habit of Abu Bakr that whenever he stood for prayer, he never glanced side-ways till he had finished it, but when Abu Bakr observed that the clapping was not coming to an end, he looked and saw the Prophet standing behind him.

The Prophet beckoned him to carry on by waving his hand. Abu Bakr stood there for a while, thanking Allah for the saying of the Prophet and then he retreated, taking his steps backwards. When the Prophet saw that, he went ahead and led the people in prayer. When he finished the prayer, he said, “O Abu Bakr! What prevented you from carrying on with the prayer after I beckoned you to do so?” Abu Bakr replied, “It does not befit the son of Abi Quhafa to lead the Prophet in prayer.” Then the Prophet said to the people, “If some problem arises during prayers, then the men should say, Subhan Allah!; and the women should clap.” (See Hadith No. 652, Vol. 1)