Bear well in mind that all misery and evil stem from ill-thinking. This is why Allah the Exalted has strictly forbidden this and states:
اِنَّ بَعۡضَ الظَّنِّ اِثۡمٌ
For suspicion in some cases is a sin.
If the Muslim clergy had not thought ill of me, and if they had listened to my words with sincerity and steadfastness, and if they read our books and stayed with me to witness our circumstances, they would not have cast aspersions on me as they do. However, when they failed to give due regard to this instruction of God Almighty and did not act upon it, they thought ill of me and thought ill of my community as well; and thus, began to raise objections and allegations against me. Some even wrote, most insolently, that we are a group of atheists, and that we do not offer the Prayers and do not fast, and so on and so forth. Now, if they had refrained from this ill-thinking, they would not have to suffer the curse of falsehood and would be saved.
I truthfully declare that ill-thinking is a most evil affliction. It destroys a person’s faith and throws them far away from sincerity and truth. It turns friends into enemies. In order to attain the excellence of the Truthful (Siddiqin), it is necessary to strictly refrain from ill-thinking. If one begins to think unfavourably about a certain person, one ought to seek forgiveness from God profusely and make supplications before God Almighty, so that one may be saved from this sin and the evil consequences which follow through from ill-thinking. One must not deem this a light matter, for it is a terrible illness, which will cause a man to perish in the swiftest of ways.
In short, ill-thinking destroys a man. This is to such an extent that when the hell-bound are cast into hell, Allah the Exalted will say that your sin was that you thought ill of Allah Almighty. There are certain people who believe that Allah the Exalted will forgive the wrongdoers but punish the righteous. This is also to think ill of God Almighty because it contradicts His attribute of justice and implies that virtue and its outcomes—as appointed by God in the Holy Quran—have been wasted, as though they were futile. Bear in mind, therefore, that the ultimate end of ill-thinking is hell, so do not consider this to be a minor illness. For ill-thinking results in despair, despair results in transgression, and transgression results in hell. It is a thing that cuts at the root of truth and sincerity (sidq). Therefore, abstain from this vice and offer supplications to attain the excellences of one who is Truthful (Siddiq).
(Malfuzat [English], Vol. 2, pp. 86-87)