Sahih Muslim 411 a
Anas b. Malik reported:
The Apostle of Allah (ﷺ) fell down from a horse and his right side was grazed. We went to him to inquire after his health when the time of prayer came. He led us in prayer in a sitting posture and we said prayer behind him sitting, and when he finished the prayer hesaid: The Imam is appointed only to be followed; so when he recites takbir, you should also recite that; when he prostrates, you should also prostrate; when he rises up, you should also rise up, and when he said" God listens to him who praises Him," you should say:" Our Lord, to Thee be the praise," and when he prays sitting, all of you should pray sitting.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَقُتَيْبَةُ بْنُ سَعِيدٍ، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَعَمْرٌو النَّاقِدُ وَزُهَيْرُ بْنُ حَرْبٍ وَأَبُو كُرَيْبٍ جَمِيعًا عَنْ سُفْيَانَ، - قَالَ أَبُو بَكْرٍ حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، - عَنِ الزُّهْرِيِّ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، يَقُولُ سَقَطَ النَّبِيُّ صلى الله عليه وسلم عَنْ فَرَسٍ فَجُحِشَ شِقُّهُ الأَيْمَنُ فَدَخَلْنَا عَلَيْهِ نَعُودُهُ فَحَضَرَتِ الصَّلاَةُ فَصَلَّى بِنَا قَاعِدًا فَصَلَّيْنَا وَرَاءَهُ قُعُودًا فَلَمَّا قَضَى الصَّلاَةَ قَالَ ‏
"‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ ‏.‏ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ ‏.‏ وَإِذَا صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا أَجْمَعُونَ ‏"‏ ‏.‏