Saying Amin

It is narrated from Mujahid (may Allah have mercy on him) that a Jew passed by some people who were in a mosque while they were saying "Amin" (at the end of a supplication). The Jew said (on seeing what they did): "By the One who taught you 'Amin' (O Allah, accept our supplication), you are certainly upon the truth."
The chain of this report is jayyid (good), and Imam Musaddad recorded it in his Musnad.

It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I have been given three qualities: prayer in rows (in congregation); I have been given the gift of the salam, which is the greeting of the people of Paradise; and I have been given 'Amin' (O Allah, accept). It was not given to anyone among those before you, except that Allah, the Blessed and Exalted, gave it to Sayyiduna Harun (peace be upon him), for Sayyiduna Musa (peace be upon him) would supplicate and Sayyiduna Harun (peace be upon him) would say Amin."
I (Ibn Hajar) say: Imam Ibn Khuzaymah recorded it in his Sahih through another chain, also from Zarbi, but there he called him the son of the daughter of al-Hurr.
I (Ibn Hajar) say: This narration is not established, because the narrator Zarbi is weak.

Ibn Umm al-Husayn narrates from his mother, Sayyidah Umm al-Husayn (may Allah be pleased with her), that she prayed behind the Prophet (peace and blessings be upon him), and she heard him (peace and blessings be upon him) recite 'Master of the Day of Judgement' [Surah al-Fatihah: 4]; then, when he (peace and blessings be upon him) completed the recitation of 'nor of those who have gone astray' [Surah al-Fatihah: 7], he (peace and blessings be upon him) said 'Amin', so that she heard it clearly although she was in the women's row (at the back of the mosque).
In the chain of this hadith, Isma'il bin Muslim al-Makki is a weak narrator; however, the imam saying Amin aloud in the audible prayers is established by many other reliable and authentic hadith.