It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I prayed behind the Prophet (peace and blessings be upon him). When he (peace and blessings be upon him) said 'Not of those who have earned [Your] anger, nor of those who are astray', he said 'Amin' and drew out his voice with it."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18257