Sahih Muslim Book - 4, Hadith - 930

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا الْمُغِيرَةُ يَعْنِي الْحِزَامِيَّ عَنْ أَبِي الزِّنَادِ عَنْ الْأَعْرَجِ عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِنَّمَا الْإِمَامُ لِيُؤْتَمَّ بِهِ فَلَا تَخْتَلِفُوا عَلَيْهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا أَجْمَعُونَ.

It was narrated from Abu Hurairah that the Messenger of Allah (s.a.w) said: "The Imam is appointed to be followed, so do not differ from him. When he says the Takbir, then say the Takbir; when he bows, then bow; when he says: 'Sami'a Allahu liman hamidah,' then say 'Allahumma Rabbana lakal-hamd,' and when he prostrates then prostrate. And if he prays sitting, then you should all pray sitting."