Musnad Abu Ya'la al-Mawsili 155:69

Ya'qub ibn Mahan narrated to us, Hushaym narrated to us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said: "The Imam was only appointed to be followed. So when he says 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar'. When he bows, then bow. And when he says 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say 'Rabbana wa lakal-hamd' (Our Lord, and to You is the praise). If he prays standing, then pray standing, and if he prays sitting, then pray sitting, all of you."