عن أبي هريرة رضي الله عنه مرفوعاً: «إذا أَمَّنَ الإمام فأمنوا، فإنه من وافق تأمينه تأمين الملائكة: غفر له ما تقدم من ذنبه».
The Prophet (may Allah's peace and blessings be upon him) ordered us to say 'amen' when the Imām says it because this is the time when the angels say 'amen', and whoever says it at the same time as the angels his previous sins will be forgiven.
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