Sahih Bukhari Volume 2, Book 20, Hadith Number 215.
Narated 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.”) (See Hadith No. 656 Vol. 1).