Sahih Bukhari Volume 1, Book 12, Hadith Number 701

Narated By Abu Huraira : The Prophet said, “The Imam is to be followed. Say the Takbir when he says it; bow if he bows; if he says ‘Sami a-l-lahu Liman hamida’, say, ‘ Rabbana wa-laka-l-hamd’, prostrate if he prostrates and pray sitting altogether if he prays sitting.”