Sahih Muslim Book 4, Hadith Number 820.

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 reported: The Messenger of Allah (may peace be upon him) rode a horse and fell down from it and his right side was grazed, and the rest of the Hadith is the same, and (these words) are found in it: “When he (the Imam) says prayer in an erect posture, you should also say it in an erect posture.”