Sahih Muslim Book 4, Hadith Number 817.

Chapter : The reciting of Tasmi’ (Allah listens to him who praises Him) Tahmid (O our Lord, for Thee is the praise), and Timin (Amin).

Anas b. Malik reported: The Apostle of Allah (may peace be upon him) fell down from a horse and his right side was grazed. We went to him to inquire after his health when the time of prayer came. He led us in prayer in a sitting posture and we said prayer behind him sitting, and when he finished the prayer he said: The Imam is appointed only to be followed; so when he recites takbir, you should also recite that; when he prostrates, you should also prostrate; when he rises up, you should also rise up, and when he said: “God listens to him who praises Him,” you should say: “Our Lord, to Thee be the praise,” and when he prays sitting, all of you should pray sitting.