Sahih Bukhari Volume 2, Book 18, Hadith 1115

حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا ابْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ سَقَطَ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ فَرَسٍ فَخُدِشَ ـ أَوْ فَجُحِشَ ـ شِقُّهُ الأَيْمَنُ، فَدَخَلْنَا عَلَيْهِ نَعُودُهُ، فَحَضَرَتِ الصَّلاَةُ فَصَلَّى قَاعِدًا فَصَلَّيْنَا قُعُودًا وَقَالَ ‏"‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ، فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا رَكَعَ فَارْكَعُوا، وَإِذَا رَفَعَ فَارْفَعُوا، وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ‏.‏ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ ‏"‏‏

Narrated By Anas bin Malik : Allah's Apostle (p.b.u.h) fell down from a horse and his right side was either injured or scratched, so we went to inquire about his health. The time for the prayer became due and he offered the prayer while sitting and we prayed while standing. He said, "The Imam is to be followed; so if he says Takbir, you should also say Takbir, and if he bows you should also bow; and when he lifts his head you should also do the same and if he says: Sami'a-l-lahu Liman Hamidah (Allah hears whoever sends his praises to Him) you should say: Rabbana walakal-Hamd (O our Lord! All the praises are for You.")