"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)." (Using translation from Nasāʾī 921)
قَالَ رَسُولُ اللهِ ﷺ إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا قَرَأَ فَأَنْصِتُوا وَإِذَا قَالَ سَمِعَ اللهُ لِمَنْ حَمِدَهُ فَقُولُوا اللهُمَّ رَبَّنَا لَكَ الْحَمْدُ