What is the ruling on the prayer of one who forgot a prostration in the last rak'ah, then said the tasleem, whereupon one of the worshippers reminded him, so he repeated the last rak'ah and performed the prostration he had forgotten, and what is the ruling on one who did not perform the last prostration with them?

If the Imam makes a mistake in his prayer, the congregants must remind him by saying "SubhanAllah" (Glorified is Allah). The two prostrations (sujud) are a pillar of prayer; the prayer is not valid without them. Whoever forgets them, or forgets one of them, must perform what they forgot as soon as they remember it. Pillars are not forfeited by forgetfulness; they must be performed. The action of the Imam, in repeating the entire last rak'ah after being reminded, is the madhhab (school of thought) of Imam Ahmad, and Sheikh Ibn Baz chose this opinion. The other opinion is to perform only the forgotten pillar and what comes after it; this is the madhhab of Imam Shafi'i, and Sheikh Ibn Uthaymin chose this opinion. The prayer of the Imam and those who completed it with him is valid. As for whoever did not perform the prostration they missed, their prayer is not valid, and they must repeat it.