Sunan Ibn Majah Book 5, Hadith 1237

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتِ اشْتَكَى رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ فَدَخَلَ عَلَيْهِ نَاسٌ مِنْ أَصْحَابِهِ يَعُودُونَهُ فَصَلَّى النَّبِيُّ ـ صلى الله عليه وسلم ـ جَالِسًا فَصَلَّوْا بِصَلاَتِهِ قِيَامًا فَأَشَارَ إِلَيْهِمْ أَنِ اجْلِسُوا فَلَمَّا انْصَرَفَ قَالَ ‏ "‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ. فَإِذَا رَكَعَ فَارْكَعُوا. وَإِذَا رَفَعَ فَارْفَعُوا. وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا ‏"‏ ‏.‏

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.’”