Will a Muslim be held accountable for unspoken insults and sexual fantasies, and what is the interpretation of Allah’s saying: "And whether you disclose what is in your yourselves or conceal it, Allah will call you to account for it"?

If one curses his brother to himself or harbors ill will towards him, he should seek Allah's forgiveness, for this is from the whispers of Satan. There is no sin upon him as long as he does not speak or act, according to the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, Allah has forgiven my Ummah for whatever they whisper to themselves, as long as they do not act upon it or speak of it."

Similarly, for one who imagines himself having sexual intercourse with a woman unlawful to him, this is from the "adultery of the mind," and he must cease such thoughts so that they do not lead him to something worse. It becomes even more heinous if he imagines this with one of his unmarriageable kin (mahārim).