Is the prayer of a worshiper valid if he was distracted during the final Tashahhud and said nothing until the Imam said the Taslim, and what should he have done?

The prayer of a congregant who said the salaam with the imam before reciting the final tashahhud (definition) is invalid according to the Hanbalis and Shafi'is, and he must repeat it, because the final tashahhud is a pillar of the prayer, and the imam does not bear the congregant's oversight regarding obligations. As for the Malikis and Hanafis, his prayer is valid, because the final tashahhud is a Sunnah (definition) according to them, and the imam bears that oversight on his behalf.