سنن ابن ماجه

It was narrated that ‘Aishah said: “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.’”