Sahih al-Bukhari 1236
Narrated `Aisha the wife of the Prophet:
Allah's Messenger (ﷺ) during his illness prayed in his house sitting, whereas some people followed him 
standing, but the Prophet (ﷺ) beckoned them to sit down. On completion of the prayer he said, "The Imam 
is to be followed. So, bow when he bows, and raise your head when he raises his head." (See Hadith 
No. 657 Vol 1 for taking the verdict).
حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَنَّهَا قَالَتْ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم فِي بَيْتِهِ وَهُوَ شَاكٍ جَالِسًا، وَصَلَّى وَرَاءَهُ قَوْمٌ قِيَامًا فَأَشَارَ إِلَيْهِمْ أَنِ اجْلِسُوا فَلَمَّا انْصَرَفَ قَالَ ‏
"‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ، فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا رَفَعَ فَارْفَعُوا ‏"‏‏.‏