Bulugh al-Maram 430a — The Book of Prayer

Abu Hurairah Narrated from:

Allah's Messenger (ﷺ) said: "The Imam has been appointed to be followed (in the congregational prayers). So, when he utters the Takbir, utter the Takbir too, and do not utter the Takbir until he utters it. When he bows, you should bow, and do not bow until he bows. When he says, "Allah listens to him who praises him," say, "O Allah, our Rabb, to you is the praise." When he prostrates, you prostrate; and do not prostrate until he prostrates. If he prays standing, pray standing, and if he prays sitting, all of you pray sitting." [Reported by Abu Dawud and the wording is his].