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