(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