Sahih Muslim Book 4, Hadith Number 824.

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

Jabir reported: The Messenger of Allah (may peace be upon him) was ill and we said prayer behind him and he was sitting. And Abu Bakr was making audible to the people his takbir. As he paid his attention towards us he saw us standing and (directed us to sit down) with a gesture. So we sat down and said our prayer with his prayer in a sitting posture. After uttering salutation he said: You were at this time about to do an act like that of the Persians and the Romans. They stand before their kings while they sit, so don’t do that; follow your Imams. If they say prayer standing, you should also do so, and if they say prayer sitting, you should also say prayer sitting.