Sunan Ibn Majah Book 5, Hadith 1238

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ صُرِعَ عَنْ فَرَسٍ فَجُحِشَ شِقُّهُ الأَيْمَنُ فَدَخَلْنَا نَعُودُهُ وَحَضَرَتِ الصَّلاَةُ فَصَلَّى بِنَا قَاعِدًا وَصَلَّيْنَا وَرَاءَهُ قُعُودًا فَلَمَّا قَضَى الصَّلاَةَ قَالَ ‏ "‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا أَجْمَعِينَ ‏"‏ ‏.‏

It was narrated from Anas bin Malik that the Prophet (ﷺ) fell from his horse and he suffered some lacerations on his right side. We went to visit him and the time for prayer came. He led us in prayer sitting down, and we prayed behind him sitting down. When he finished the prayer he said:“The Imam is appointed to be followed. When he says Allahu Akbar, then say Allahu Akbar; when he bows, then bow; when he says Sami’ Allahu liman hamidah, then say Rabbana wa lakal-hamd; when he prostrates then prostrate; and if he prays sitting down then pray sitting down.”