Yahya related to me from Malik from Hisham ibn Urwa from his
father that A'isha, the wife of the Prophet, may Allah bless him and
grant him peace, said, "The Messenger of Allah, may Allah bless him
and grant him peace, 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 .' "
Muwatta Malik Book 8, Hadith 18