What is the ruling regarding a congregational prayer in which a worshiper abandoned the rukūʿ (bowing) and went down to prostrate, then completed the prayer with the Imam who forgot the first sitting and compensated for it with two prostrations of forgetfulness?

If a worshipper (ma'mum) lags behind the Imam by two pillars due to an excuse, he must perform them and then continue with the Imam. If he does not do so and prostrates with the Imam, that rak'ah (unit of prayer) is nullified, and he must perform an additional rak'ah after the Imam says the taslim (final salutation). If he leaves the prayer without performing that rak'ah, he is obligated to repeat the entire prayer.