What is the ruling on cursing someone inwardly without him knowing it?

A Muslim should turn away from the ignorant, suppress their anger, and forgive those who have wronged them. This is part of the Islamic ethics to which Allah has guided in the Holy Quran. Allah Almighty says: "Hold to forgiveness, command what is good, and turn away from the ignorant," and "who suppress anger and pardon people." Nevertheless, it is their right to respond to those who revile them and to retaliate against those who aggress against them. Allah Almighty says: "The recompense of an evil deed is an evil deed like it." As for what a person says to themselves, it carries no ruling unless they speak it or act upon it, due to the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah has pardoned for my Ummah whatever occurs to themselves, as long as they do not act upon it or speak of it."