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) enter into the prayer, and when he (peace and blessings be upon him) finished Surah al-Fatihah he said 'Amin' three times."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18258