Lu'lu' wal-Marjan

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings of Allah be upon him) said: The Imam is appointed so that he may be followed, so when he says the takbir, you should also say it; when he bows (in ruku‘), you should also bow; and when he says "Sami‘ Allahu liman hamidah," you should say "Rabbana wa lakal hamd"; and when he prostrates, you should also prostrate; and when he prays sitting, then all of you should also pray sitting.