«إنما جعل الإمام ليؤتم به فإذا كبر فكبروا وإذا قرأ فأنصتوا وإذا قال سمع الله لمن حمده فقولوا: ربنا لك الحمد»
The Prophet (peace and blessings be upon him) said:
"The imam has been appointed only so that he may be followed. So when he says the takbir, you too say the takbir; when he recites, listen in silence (attentively); and when he says 'Sami'a Allahu liman hamidah' (Allah hears the one who praises Him), then say 'Rabbana lakal-hamd' (Our Lord, to You belongs all praise)."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 2358