Malik Muwatta: Hadith 306

Yahya related to me from Malik from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet (ﷺ) said, "The Messenger of Allah (ﷺ) prayed while he was ill. He prayed sitting, and some people prayed behind him standing, and he indicated to them to sit down. When he left, he said, 'The imam is only appointed to be followed. When he goes into ruku, go into ruku and when he rises, rise and if he prays sitting, pray sitting .' "