What is the ruling on the prayer of a congregant who joined the Imam during the prostration of the first rak'ah, then completed his prayer after the taslim of the Imam who prayed three rak'ahs with one tashahhud due to forgetfulness? And what is the ruling on the prayer of the Imam and the congregants who joined him at the opening takbir or during the first rak'ah?

If the Imam prayed Dhuhr as three rak'ahs and a long time elapsed after he concluded, then he and those who concluded with him after three rak'ahs must repeat the prayer, according to Ibn Qudamah: "If a long time elapses, or his ablution is invalidated, he restarts the prayer." As for the latecomer (masbuq) who completed his prayer and concluded after four rak'ahs, his prayer is valid; because the invalidation of the Imam's prayer does not affect the prayer of the congregants, and because he feared Allah as much as he could by doing what he was able to do, as Allah Almighty says: "So fear Allah as much as you are able."