(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