Musnad Ahmad 30:1037

The Messenger of Allah, may Allah bless him and grant him peace, said: "The Imam was only appointed to be followed. So do not differ from him. When he says the Takbir, then say the Takbir; when he bows, then bow; and when he says, 'Allah hears those who praise Him,' then say, 'O Allah, our Lord, to You is the praise.' When he prostrates, then prostrate; and when he prays sitting, then pray sitting, all of you."