Sahih Muslim Book 4, Hadith 104
When the Messenger of Allah (ﷺ) suffered from illness of which he died, and in the hadith transmitted by Ibn Mus-hir, the words are: The Messenger of Allah (ﷺ) was brought till he was seated by his (Abu Bakr's) side and the Apostle (ﷺ) led the people in prayer and Abu Bakr was making takbir audible to them, and in the hadith transmitted by 'Isa the (words are):" The Messenger of Allah (ﷺ) sat and led the people in prayer and Abu Bakr was by his side and he was making (takbir) audible to the people."
When the Messenger of Allah (ﷺ) was confined to bed, Bilal came to him to summon him to prayer. He (the Holy Prophet) said: Ask Abu Bakr to lead the people in prayer. She ('A'isha) reported: I said: Messenger of Allah, Abu Bakr is a tenderhearted man, when he would stand at your place (he would be so overwhelmed by feelings) that he would not be able to make the people hear anything (his recitation would not be audible to the followers in prayer). You should better order Umar (to lead the prayer). He (the Holy Prophet) said: Ask Abu Bakr to lead people in prayer. She ('A'isha) said: I asked Hafsa to (convey) my impression to him (the Holy Prophet) that Abu Bakr was a tenderhearted man, so when he would stand at his place, he would not be able to make the people bear anything. He better order Umar. Hafsa conveyed this (message of Hadrat 'A'isha) to him (the Holy Prophet). The Messenger of Allah (ﷺ) said: (You are behaving) as if you are the females who had gathered around Yusuf. Order Abu Bakr to lead the people in prayer. She ('A'isha) reported: So Abu Bakr was ordered to lead the people in prayer. As the prayer began, the Messenger of Allah (may peace he upon him) felt some relief; he got up and moved supported by two persons and his feet dragged on earth (due to excessive weakness). 'A'isha reported: As he (the Holy Prophet) entered the mosque. Abu Bakr perceived his (arrival). He was about to withdraw, but the Messenger of Allah (ﷺ) by the gesture (of his hand) told him to keep standing at his place. The Messenger of Allah (ﷺ) came and seated himself on the left side of Abu Bakr. She ('A'isha) reported: The Messenger of Allah (ﷺ) was leading people in prayer sitting. Abu Bakr was following the prayer of the Apostle (ﷺ) in a standing posture and the people were following the prayer of Abu Bakr.
The Messenger of Allah (ﷺ) ordered Abu Bakr that he should lead people in prayer during his illness, and he led them in prayer. `Urwa said: The Messenger of Allah (ﷺ) felt relief and went (to the mosque) and Abu Bakr was leading the people in prayer. When Abu Bakr saw him he began to withdraw, but the Messenger of Allah (ﷺ) signaled him to remain where he was. The Messenger of Allah (ﷺ) sat opposite to Abu Bakr by his side. Abu Bakr said prayer following the prayer of the Messenger of Allah (ﷺ), and the people said prayer following the prayer of Abu Bakr.