Sahih Muslim Book 4, Hadith Number 834.

Chapter : The Muqtadi (follower) should strictly follow the Imam in prayer.

‘Aisha, the wife of the Apostle (may peace be upon him), said: When the Messenger of Allah (may peace be upon him) fell ill and his illness became serious, he asked permission from his wives to stay in my house during his illness. They gave him permission to do so. He stepped out (of ‘Aisha’s apartment for prayer) supported by two persons. (He was so much weak) that his feet dragged on the ground and he was being supported by ‘Abbas b. ‘Abd al-Muttalib and another person. ‘Ubaidullah said: I informed ‘Abdullah (b. ‘Abbas) about that which ‘Aisha had said. ‘Abdullah b. ‘Abbas said: Do you know the man whose name ‘Aisha did not mention? He said: No. Ibn ‘Abbas said: It was ‘Ali.