Sunan Ibn Majah Book 5, Hadith 846

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ، عَنِ ابْنِ عَجْلاَنَ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏"‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا قَرَأَ فَأَنْصِتُوا وَإِذَا قَالَ ‏{غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ}‏ فَقُولُوا آمِينَ وَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ ‏.‏ فَقُولُوا اللَّهُمَّ رَبَّنَا وَلَكَ الْحَمْدُ وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا أَجْمَعِينَ ‏"‏ ‏.‏

It was narrated that Abu Hurairah said:“The Messenger of Allah (ﷺ) said: ‘The Imam has been appointed to be followed, so when he says Allahu Akbar, then say Allahu Akbar, when he recites, then listen attentively; when he says: Not (the way) of those who earned Your anger, nor of those who went astray,[1:7] then say Amin; when he bows then bow; when he says Sami’ Allahu liman hamidah (Allah hears those who praise Him), then say Allahumma Rabbana wa lakal-hamd (O Allah, our Lord, to You is the praise);” when he prostrates then prostrate; and if he prays sitting down then all of you pray sitting down.’”