48. To think ill and attribute wrong motives to others' actions is a diseased and distorted attitude in man which destroys the quality of his faith and righteousness and consumes it as rapidly as blazing fire consumes tinder. When such diseased people make prophets of God the target of their insinuations and think ill of them, God becomes their enemy and stands up in defence of His prophets. He guards the honour of His dear ones with such jealousy as is unparalleled. When I was maligned and assailed in different ways, the same protective jealousy of God became operative in my defence.
(Al Wasiyyat, footnote: Roohani Khaza'in Vol. 20, p. 317)
49. I tell you truly that the habit of thinking ill of others is a great affliction which destroys a person's faith, flings him away from truth and turns his friends into foes. In order to attain the excellent virtues of a Siddeeq (Siddeeq is a term used by the Holy Quran indicating a spiritual rank next to that of prophets) it is necessary that a person should altogether shun the habit of thinking ill of others. If inadvertently he happens to have thought ill of others, he should forthwith repent and seek forgiveness repeatedly and pray to God that he may be protected from committing this folly again and be saved from its consequences. This spiritual malady should not be taken lightly. It is a highly dangerous disease which speedily destroys its victim.
(Malfoozat Vol. 1: p 356)