Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

"I entered upon Aishah and said: 'Will you not tell me about the sickness of the Messenger of Allah (saw)?' She said: 'When the Messenger of Allah (saw) became seriously ill, he said: "Have the people prayed?" We said: "No, they are waiting for you, O Messenger of Allah." He said: "Put some water in a tub for me." We did that and he performed Ghusl, then he tried to get up but he fainted. Then he came to us and said: "Have the people prayed?" We said: "No, they are waiting for you, O Messenger of Allah." He said: "Put some water in a tub for me." We did that and he performed Ghusl, then he tried to get up but he fainted. Then for the third time he said the same thing. She said: The people were in the Masjid, waiting for the Messenger of Allah (saw) to lead the Isha prayer. The Messenger of Allah (saw) sent word to Abu Bakr, telling him to lead the people in prayer, so the messenger came to him and said: "The Messenger of Allah (saw) is telling you to lead the people in prayer." Abu Bakr was a tender-hearted man, so he said: "O Umar, lead the people in prayer." But (Umar) said: "You have more right to that." So Abu Bakr led them in prayer during those days. When the Messenger of Allah (saw) felt a little better, he came with the help of two men, one of whom was Al-Abbas, to pray Zuhr. When Abu Bakr saw him, he wanted to step back, but the Messenger of Allah (saw) gestured to him not to step back. He told them (the two men) to seat him beside him, and Abu Bakr started to pray standing, and the people were following the prayer of Abu Bakr, and the Messenger of Allah (saw) was praying sitting.'" "I (Ubaidullah) entered upon Ibn Abbas and said: 'Shall I not tell you what Aishah narrated to me about the sickness of the Messenger of Allah (saw)?' He said: 'Yes.' So I told him and he did not deny any of it, but he said: 'Did she tell you the name of the man who was with Al-Abbas?' I said: 'No.' He said: 'That was Ali, may Allah honor his face.'" (Sahih)