It is narrated from Abu Salamah and Sa'id that Abu Hurayrah (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) finished reciting Surah al-Fatihah, he would raise his voice and say: "Amin."
Ali bin Umar al-Hafiz (al-Daraqutni) said: This chain is hasan (good).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2452