Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 603
Narated By AbuHurayrah : The Prophet (pbuh) said: The imam is appointed only to be followed; when he says "Allah is most great," say "Allah is most great" and do not say "Allah is most great" until he says "Allah is most great." When he bows; bow; and do not bow until he bows. And when he says "Allah listens to him who praise Him," say "O Allah, our Lord, to Thee be the praise."
The version recorded by Muslim goes: "And to Thee be the praise: And when he prostrate; and do not prostrate until he prostrates. When he prays standing, pray standing, and when he prays sitting, all of you pray sitting.