What is the ruling on the prayer of a man who left the congregational prayer in the third رکعة to kill a scorpion, then caught up with the Imam at the beginning of the fourth رکعة and concluded the prayer with him, then made up only the third رکعة? And what is the correct course of action in this situation?

What the man did by killing the scorpion during prayer is permissible, based on the saying of the Prophet, peace and blessings be upon him: "Kill the two black ones during prayer: the snake and the scorpion."

And his making up the third rak'ah that he missed due to killing the scorpion is correct, provided he did not exit the prayer by intention or by another nullifier; based on the saying of the Prophet, peace and blessings be upon him: "When you hear the iqamah, walk to prayer with tranquility and dignity, and do not rush. Whatever you catch, pray, and whatever you miss, complete."

However, if he had exited the prayer by intention or by a nullifier occurring, then he must restart the prayer and it is not permissible for him to build upon what he had already prayed before the interruption.