Musannaf Ibn Abi Shayba 3:5584

Waki' narrated to us, he said: Isra'il narrated to us, from Abu Ishaq, from Abu Maysarah, that Gabriel (peace be upon him) recited the Opening of the Book (Al-Fatiha) to the Prophet (may Allah bless him and grant him peace). When he said, "Nor of those who go astray," he said, "Say: Amin." So he said, "Amin."