It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the imam says the takbir, say the takbir; when he bows, bow; when he prostrates, prostrate; when he raises his head from bowing, raise yours; and if he prays sitting, then all of you pray sitting."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7656