Sayyiduna Bilal (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) about this, and he (peace and blessings be upon him) said: "Do not get ahead of me in saying 'Amin'."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2298