Sahih Muslim Book - 4, Hadith - 926

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ عَنْ هِشَامٍ عَنْ أَبِيهِ عَنْ عَائِشَةَ قَالَتْ اشْتَكَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَدَخَلَ عَلَيْهِ نَاسٌ مِنْ أَصْحَابِهِ يَعُودُونَهُ فَصَلَّى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ جَالِسًا فَصَلُّوا بِصَلَاتِهِ قِيَامًا فَأَشَارَ إِلَيْهِمْ أَنْ اجْلِسُوا فَجَلَسُوا فَلَمَّا انْصَرَفَ قَالَ إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا.

It was narrated that 'Aishah said: "The Messenger of Allah (s.a.w) was sick and some of his Companions entered upon him to visit him. The Messenger of Allah (s.a.w) offered prayers sitting down, and they followed him in prayer, standing up, but he gestured to them to sit down, so they sat down. When he had finished he said: 'The Imam is appointed to be followed; when he bows, then bow; when he rises, then rise; and if he prays sitting then pray sitting."'