(a) It is narrated from Abu Uthman al-Nahdi that Sayyiduna Bilal (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not go ahead of me in saying Amin."
(b) 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 get ahead of me in saying Amin."
(c) This hadith points to the fact that Sayyiduna Bilal (may Allah be pleased with him) used to say Amin before the Prophet (peace and blessings be upon him) said Amin, so he (peace and blessings be upon him) said: "Do not get ahead of me in saying Amin."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2300