Is having ill thoughts about a Muslim forbidden if one speaks of them, or is the doer sinful even if he does not speak of them? What about having ill thoughts about Allah? And is the saying of Sufyan al-Thawri sufficient to settle this matter?

Scholars have differed on the issue of entertaining ill thoughts (su' az-zann). Some have absolutely prohibited it, even if the person does not vocalize what they have thought, citing the general nature of texts that forbid ill thoughts, such as the verse: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." (Quran 49:12), and the hadith: "Beware of suspicion, for suspicion is the most untruthful of speech."

Others, such as Sufyan al-Thawri, are of the view that there are two types of suspicion: one is sinful, which is to suspect and then speak of it; and the other is not sinful, which is to suspect but not speak of it. This is the view preferred by Al-Khadimi. As for Al-Qurtubi, he understood the prohibition to apply to suspicion that involves accusation without any compelling reason, especially concerning those whose outward appearance is righteous.

As for entertaining ill thoughts about Allah Almighty, it is a major sin but not disbelief (kufr). It has been narrated from the Prophet, peace and blessings be upon him, that he said: "The greatest of major sins is entertaining ill thoughts about Allah Almighty." It is also a characteristic of hypocrites.