What is the ruling if the Imam forgets or omits saying 'Amin'?

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If the Imam forgets 'Amin,' the follower must say it loudly to remind the Imam so that he may say it, as it is a verbal Sunnah that the follower supplies if the Imam neglects it, similar to the Isti'adhah (seeking refuge in Allah). If the Imam conceals the 'Amin,' the follower should say it audibly, based on the previous reasoning. If the Imam omits 'Amin,' whether by forgetfulness or intentionally, and begins reciting the next Surah, he should not say it, as its prescribed time has passed.