What is the ruling on insulting the soul by saying: "May the soul of so-and-so be cursed"?

The soul is the self, so cursing a person's soul is cursing the person himself, and this is one of the major sins. The Prophet, peace and blessings be upon him, forbade cursing a believer, and whoever curses a believer is like killing him. If a curse is uttered, and it does not find someone who deserves it, it returns to the one who uttered it. Therefore, whoever has done this must repent, seek forgiveness, and resolve not to return to it.