It is narrated from Ata (may Allah have mercy on him) that he states: I heard from the imams appointed by Abdullah bin Zubair (may Allah be pleased with them both) and the imams after them that they would say Ameen, and their followers would also say Ameen, to the extent that the mosque would resound.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 217