Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him), while teaching us (about prayer), used to say: "When the imam says the takbir, you should also say the takbir, and when he recites « غَيْرِالْمَغْضُوبِعَلَيْهِمْوَلَاالضَّالِّينَ », you should say Ameen."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1582