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 wasnarrated that ‘Aishah said: “The Messenger of Allah (PBUH) fellill and some of his Companions came to visit him. The Messenger ofAllah (PBUH) performed prayer while sitting down, and they prayedbehind him standing up. He gestured them to sit down, and when hefinished he said: ‘The Imam is appointed to be followed. When hebows,then bow; when he stands up again, then stand up, and if heprayssitting down then pray sitting down.’” Translation Not Available

Book Ref: Sunan Ibn Majah Book 5 Hadith 1237
Web Ref:  Sunan Ibn Majah Vol 1 Book 5 Hadith 1237