It was narrated from Anas that:

The Prophet ﷺ fell from a horse onto his right side, and they entered upon him to visit him. The time for prayer came, and when he had finished praying he said: "The Imam is appointed to be followed, so when he bows, then bow, and when he stands up, then stand up, and when he says: 'Sami' Allahu liman hamidah (Allah hears the one who praises Him)' then say: 'Rabbana wa lakal-hamd (Our Lord, and to You be the praise).'"

أَخْبَرَنَا هَنَّادُ بْنُ السَّرِيِّ، عَنِ ابْنِ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ ﷺ سَقَطَ مِنْ فَرَسٍ عَلَى شِقِّهِ الأَيْمَنِ فَدَخَلُوا عَلَيْهِ يَعُودُونَهُ فَحَضَرَتِ الصَّلاَةُ فَلَمَّا قَضَى الصَّلاَةَ قَالَ " إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ " .