Jami` at-Tirmidhi 361

Anas bin Malik narrated:"Allah's Messenger fell from a horse and got injured, so he led Salat sitting and we also offered Salat sitting. When he completed the Salat he said: "The Imam is appointed to be followed; when he says the Takbir then say the Takbir, when he bows, then bow, and when he raises his head, then raise your heads. When he says: "Sami' Allahu liman hamidah (Allah listens to those who praise him)" then say: "Rabbana wa lakal-hamd. (O our Lord! And all praise is Yours.)" and when he prostrates, then prostrate, and when he performs Salat sitting, then pray sitting altogether