(a) It is narrated from Sayyiduna Bilal (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not say Amin before me."
(b) The Shaykh (may Allah have mercy on him) said: Sayyiduna Bilal (may Allah be pleased with him) used to say Amin before the Messenger of Allah (peace and blessings be upon him), so he (peace and blessings be upon him) said: "Do not precede me in saying Amin," just as (elsewhere) he (peace and blessings be upon him) said: "When the imam says Amin, then you say Amin."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2441