Is the prayer valid for an Imam who prostrated once in a rak'ah, then completed it after being reminded by the congregants, and did not sit between the two prostrations?

If a worshiper sits before standing, even if it is a session of rest (jalsa istiraha), it is sufficient for the sitting between the two prostrations, and their prayer is valid. However, if they did not sit, they must sit and then prostrate after sitting between the two prostrations. If they do not sit—even for a brief rest—they have omitted an obligatory pillar, which is the sitting between the two prostrations, and whoever omits it, their prayer is invalid.