Sahih Muslim Book 4, Hadith Number 822.

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

‘Aisha reported: The Messenger of Allah (may peace be upon him) fell ill and some of his Companions came to inquire after his health. The Messenger of Allah (may peace he upon him) said prayer sitting, while (his Companions) said it (behind him) standing. He (the Holy Prophet) directed them by his gesture to sit down, and they sat down (in prayer). After finishing the (prayer) lie (the Holy Prophet) said: The Imam is appointed so that he should be followed, so bow down when lie bows down, and rise rip when he rises up and say (prayer) sitting when he (the Imam) says (it) sitting.