Is it permissible to inwardly curse someone in one's heart, and is that considered a sin?

The ruling on cursing and insulting varies depending on the reason for its occurrence:

1. If it is justified: It is permissible for the wronged person to retaliate with cursing and insulting without exceeding limits. However, forgiveness and overlooking are preferable, as Allah Almighty says: "Allah does not like the public utterance of evil, except by one who has been wronged," and the Prophet, peace be upon him, said: "The two who are cursing each other, what they say is upon the one who initiated it, unless the wronged person exceeds the limit."

2. If it is unjustified: It is considered one of the manifest prohibitions and a slip of the tongue, as the Prophet, peace be upon him, said: "Insulting a Muslim is wickedness, and fighting him is disbelief."

3. If it is a thought or inner monologue: Thoughts and ideas are forgiven as long as they are not acted upon or spoken, as the Prophet, peace be upon him, said: "Indeed, Allah has forgiven my Ummah for what they think within themselves, as long as they do not act upon it or speak of it." However, one must beware of harboring evil, as it is among the deeds of the heart for which a servant will be held accountable.