Volume 1, Book 12, Number 701:
Narrated 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.”