It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: I saw the Prophet (peace and blessings be upon him) recite "Not of those who have earned [Your] anger, nor of those who are astray" [Al-Fatihah: 7], and he (peace and blessings be upon him) said "Amin" and drew out his voice with it.
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18254