Sahih Muslim Book 4, Hadith 109
The Apostle of Allah (ﷺ) did not come to us for three days. When the prayer was about to start. Abu Bakr stepped forward (to lead the prayer), and the Messenger of Allah (ﷺ) lifted the curtain. When the face of the Messenger of Allah (ﷺ) became visible to us, we (found) that no sight was more endearing to us than the face of the Messenger of Allah (ﷺ) as it appeared to us. The Apostle of Allah (ﷺ) with the gesture of his hand directed Abu Bakr to step forward (and lead the prayer). The Apostle of Allah (ﷺ) then drew the curtain, and we could not see him till he died.
This hadith is narrated on the authority of Anas b. Malik by another chain of transmitters.
When the Messenger of Allah (ﷺ) became ill and illness became serious he ordered Abu Bakr to lead the people in prayer. Upon this 'A'isha 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 (ﷺ).