Abu Huraira reported Allah's Messenger ﷺ as saying, "Do not try to go ahead of the imam. When he says ‘Allah is most great’ say ‘Allah is most great’; when he says ‘Nor of those who err’ (i.e. the end of sura 1) say ‘Amen’; when he bows bow; and when he says ‘Allah listens to him who praises Him’ say, ‘O Allah, our Lord, to Thee be the praise’." (Bukhari and Muslim, but Bukhari did not mention "and when he says, ‘Nor of those who err”’.)