Maliks Muwatta Book 8, Hadith Number 17.
Section : Prayer Behind an Imam when He Prays Sitting.
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.”