Sahih Muslim Book 4, Hadith Number 833.

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

‘Aisha reported: It was in the house of Maimuna that the Messenger of Allah (may peace be upon him) first fell ill. He asked permission from his wives to stay in her (‘Aisha’s) house during his illness. They granted him permission. She (‘Aisha) narrated: He (the Holy Prophet) went out (for prayer) with his hand over al-Fadl b. ‘Abbas and on the other hand there was another person and (due to weakness) his feet dragged on the earth. ‘Ubaidullah said: I narrated this Hadith to the son of ‘Abbas (‘Abdullah b. ‘Abbas) and he said: Do you know who the man was whose name ‘Aisha did not mention? It was ‘Ali.