It is narrated from Ata (may Allah have mercy on him) who states: I heard the Imams and mentioned Ibn Zubair (may Allah be pleased with them both) and the era after him, that the (Imam) would say Ameen and their followers would also say Ameen, to the extent that the mosque would resound.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 218