It is narrated from Abu Rafi' (may Allah have mercy on him) that Sayyiduna Abu Hurayrah (may Allah be pleased with him) was the mu'adhdhin of Marwan bin al-Hakam. It had been stipulated that he (Marwan) would not say: 'Nor of those who go astray' [Surah al-Fatihah: 7] until he knew that Sayyiduna Abu Hurayrah (may Allah be pleased with him) had joined the row. So when Marwan said: 'Nor of those who go astray' [Surah al-Fatihah: 7], Sayyiduna Abu Hurayrah (may Allah be pleased with him) would say "Amin," drawing out his voice. He (may Allah be pleased with him) used to say: "When the Amin of the people of the earth coincides with the Amin of the inhabitants of the heaven, the people of the earth are forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2453
(a) It is narrated from Ata (may Allah have mercy on him) that he said: I heard Ibn al-Zubayr (may Allah be pleased with him) and the imams after him; they would say Amin and those behind them would also say Amin, until the mosque resounded.
(b) It is also reported from Ibn Umar (may Allah be pleased with them both) that he would say Amin aloud, whether he was the imam or a follower.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2454
(a) It is narrated from Ata (may Allah have mercy on him) that he said: In this mosque I saw two hundred Companions of the Prophet (peace and blessings be upon him); when the imam said: 'Not of those who have earned (Your) anger, nor of those who go astray' [Surah al-Fatihah: 7], they would say Amin, and I would hear the resounding of their Amin.
(b) Ishaq al-Hanzali narrated it from Ali bin al-Hasan, and in it is that the Companions (may Allah be pleased with them) raised their voices with Amin.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2455