What is the ruling on the prayer of one who completed his prayer individually after the Imam made a mistake, then the Imam reconsidered and performed the prostration of forgetfulness (sujood as-sahw)?

If you did not realize the Imam's prostration until after you stood up to complete the prayer, then your continuation of your prayer is valid. However, you should have performed the prostration of forgetfulness (sujud al-sahw) after completing your prayer, because the latecomer (masbuq) is obligated to perform it even if he did not experience its cause. If the worshipper (ma'mum) stood up to make up what he missed, and the Imam prostrated after his tasleem (salutation), then the worshipper should not return to the prostration if he has already stood upright. And if he has begun reciting (Qur'an), it is forbidden for him to return; rather, he should complete his prayer and prostrate after his tasleem. Since you did not perform the prostration of forgetfulness, it is more cautious for you to repeat the prayer.