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, and when he reached "Not of those who have earned [Your] anger, nor of those who are astray" [Al-Fatihah: 7], he said "Amin" aloud.
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18251