Musnad Ahmad 30:1768

Abu Sa’d al-Saghani Muhammad ibn Maysar narrated to us, Muhammad ibn ‘Ajlan narrated to us, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The Imam is only appointed to be followed. So when he says the Takbir, say the Takbir; and when he recites, listen silently; and when he says: {Not of those who have evoked [Your] anger or of those who are astray} [Al-Fatihah: 7], say: 'Amin.' And when he bows, bow; and when he says: 'Allah hears those who praise Him,' say: 'Our Lord, and to You is the praise.' And if he prays sitting, then pray sitting, all of you."