It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I prayed with the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) began with Surah al-Fatihah, and when he completed it he said 'Amin', so that those behind him (peace and blessings be upon him) heard it."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18253