(a) Alqamah bin Wa'il narrates from his father (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) saying "Amin" aloud.
(b) The wording of the narration of Abd al-Jabbar is: "He drew out his voice with it (the Amin)," and the wording of another narration is: "He raised his voice with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2449