“The Messenger of Allah (ﷺ) fell ill and some of his Companions came to visit him. The Messenger of Allah (ﷺ) performed prayer while sitting down, and they prayed behind him standing up. He gestured them to sit down, and when he finished he said: ‘The Imam is appointed to be followed. When he bows, then bow; when he stands up again, then stand up, and if he prays sitting down then pray sitting down.’”
Sunan Ibn Majah Book 5, Hadith 435
← Previous Hadith “The Messenger of Allah (ﷺ) lagged behind (on a journey) and we reached the people when ‘Abdur Rahman bin ‘Awf had already led them in one Rak’ah of the prayer. When he realized that the Prophet (ﷺ) was there, he wanted to step back, but the Prophet (ﷺ) gestured to him that he should complete the prayer. He said: ‘You have done well, do the same in the future.’”

Sunan Ibn Majah Book 5, Hadith 434 Next Hadith → “The Imam is appointed to be followed. When he says Allahu Akbar, then say Allahu Akbar; when he bows, then bow; when he says Sami’ Allahu liman hamidah, then say Rabbana wa lakal-hamd; when he prostrates then prostrate; and if he prays sitting down then pray sitting down.”

Sunan Ibn Majah Book 5, Hadith 436