What is the ruling on insulting and cursing inwardly (to oneself) without moving the tongue, and is the doer sinful for it?

Merely thinking to oneself without moving the tongue incurs no accountability, as the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my nation for whatever their souls whisper or speak to them, as long as they do not act upon it or speak of it." However, one must strive against their self to cultivate patience, forbearance, and forgiveness, and accustom their tongue to speak good and to repel people's ill-treatment with kind words.