Musnad Abu Ya'la al-Mawsili 159:78

And with his chain of narration, that the Messenger of Allah (ﷺ) said: "The Imam was only appointed to be followed, so do not differ from him. So when he says the Takbir, then say the Takbir. And when he bows, then bow. And when he says, 'Allah hears those who praise Him,' then say, 'Our Lord, to You is the praise.' And when he prostrates, then prostrate. And when he prays sitting, then pray sitting, all of you."