It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The imam is appointed only to be followed, so do not differ from him. When he says the takbir, say the takbir; when he bows, bow; when he says 'Sami' Allahu liman hamidah', say 'Allahumma rabbana wa laka al-hamd'; when he prostrates, prostrate; and when he leads the prayer sitting, all of you pray sitting."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2277