Sahih Muslim Book 4, Hadith 88

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.