Mukhtasar Muslim (Abridged) 275 — Prohibition of preceding imam

Sidna Abu Hurairah, may Allah be pleased with him, says that the Messenger of Allah, peace and blessings be upon him, used to teach us and say: Do not do any action before the Imam; when he says the takbeer, then you say the takbeer; and when he says "wa la ad-dallin," then you say "Ameen" after him; and when he bows (for ruku‘), then you bow after him; and when he says "Sami‘ Allahu liman hamidah," then you say "Rabbana lakal hamd" after him.