Sahih Bukhari Volume 1, Book 11, Hadith Number 689

Narated By Abu Huraira : The Prophet said, “The Imam is (appointed) to be followed. So do not differ from him, bow when he bows, and say, “Rabbana-lakal hamd” if he says “Sami’a-l-lahu Liman hamida”; and if he prostrates, prostrate (after him), and if he prays sitting, pray sitting all together, and straighten the rows for the prayer, as the straightening of the rows is amongst those things which make your prayer a correct and perfect one. (See Hadith No. 657).