Hadith

Narrated Abu Huraira: 

The Prophet (sallallahu alayhi wa salam) (May peace and blessings be upon him) 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." 

{Sahih Bukhari, Book 12, Number 701}