Hazrat Hattan bin Abdullah al-Raqashi narrates that Sayyiduna Abu Musa Ash'ari (may Allah be pleased with him) led us in prayer. Then when he finished the prayer, he said that indeed the Messenger of Allah (peace be upon him) delivered a sermon to us, explained to us our Sunnahs, and taught us our prayer. He (peace be upon him) said: "When the Imam says the takbeer, you also say the takbeer, and when he recites «‏‏‏‏غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ ‎» , you say Ameen. Allah Almighty will accept your supplication." Imam Abu Bakr (may Allah have mercy on him) says that this hadith relates to the follower saying Ameen after the Imam has finished reciting Surah Al-Fatihah, even if the Imam, due to ignorance or forgetfulness, does not say Ameen.