Muwatta Malik Book 8, Hadith 17
Yahya related to me from Malik from Ibn Shihab from Anas ibn
Malik that the Messenger of Allah, may Allah bless him and grant him
peace, fell off his horse when riding, and his right side was scraped,
so he did one of the prayers sitting, and we prayed behind him
sitting. When he left, he said, "The imam is appointed to be followed.
If he prays standing, then pray standing, and when he goes into ruku,
go into ruku, and when he rises, rise, and when he says, 'Allah hears
whoever praises him,' say, 'Our Lord, praise belongs to You,' and if
he prays sitting, then all of you pray sitting."
Yahya related to me from Malik from Yahya ibn Said that a man used to lead the people in prayer in al-Aqiq (a place near Madina), and Umar ibn Abd al-Aziz sent a message to him forbidding him to do so.
Malik said, "He only forbade him because his father was not known."
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 .' "