"إنما جعل الإمام ليؤتم به فلا تختلفوا عليه وإذا كبر فكبروا وإذا ركع فاركعوا وإذا قال: سمع الله لمن حمده فقولوا: ربنا لك الحمد وإذا سجد فاسجدوا وإذا صلى جالسا ¤ فصلوا جلوسا أجمعين"
The Prophet (peace and blessings be upon him) said:
"The imam has been appointed only so that he may be followed, so do not differ from him (in his actions). When he says the takbir, say the takbir; when he bows, bow; 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); when he prostrates, prostrate; and when he prays sitting, all of you together pray sitting."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 2360