It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I saw the Messenger of Allah (peace and blessings be upon him) and prayed behind him. He (peace and blessings be upon him) began reciting 'Not of those who have earned [Your] anger, nor of those who are astray' and said 'Amin' aloud, so that I heard it."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18256