Sahih Muslim Book 4, Hadith 89
This hadith is narrated with the same chain of transmitters by Hisham b. 'Urwa.
The Messenger of Allah (ﷺ) fell ill and some of his Companions came to inquire after his health. The Messenger of Allah (ﷺ) 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) he (the Holy Prophet) said: The Imam is appointed so that he should be followed, so bow down when he bows down, and rise upp when he rises up and say (prayer) sitting when he (the Imam) says (it) sitting.
The Messenger of Allah (ﷺ) 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.