It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "When the Messenger of Allah (peace and blessings be upon him) said 'nor of those who are astray', he would say 'Amin', until he made us hear it."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18250