It was narrated that Abu Hurairah said:"The Messenger of Allah (ﷺ) said: 'The Imam is appointed to be followed, so when he says the takbir, say the takbir, and when he recites, be silent, and when he says: "Sami' Allahu liman hamidah (Allah hears those who praise Him)," say: "Allahumma rabbana lakal-hamd (Our Lord, to You be praise)

English translation · Sunan an-Nasa'i