Sahih Muslim Book 4, Hadith Number 844.
Chapter : The Muqtadi (follower) should strictly follow the Imam in prayer.
Abu Musa reported: When the Messenger of Allah (may peace be upon him) became ill and illness became serious he ordered Abu Bakr to lead the people in prayer. Upon this ‘Aisha said: Messenger of Allah, Abd Bakr is a man of tenderly feelings: when he would stand in your place (he would be so much overwhelmed -by grief that) he would not be able to lead the people in prayer. He (the Holy Prophet) said: You order Abu Bakr to lead the people in prayer, and added: You are like the female companions of Yusuf. So Abu Bakr led the prayer (during this period of illness) in the life of the Messenger of Allah (may peace be upon him).