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Negative Thoughts (Suspicions)
Some assumptions are not permissible, such as holding a bad opinion about someone who manifests righteous behaviour. This means that your heart is convinced and you have judged him based on your heart's suspicions without proof that warrants such an assumption. There is nothing wrong with having doubts about someone or having a bad opinion of him, if it is based on sound reasoning and is not baseless.

Imam Mawlūd speaks of something that is very easy to have but is harmful to brotherhood and injurious to one's own spiritual growth. It is having a bad opinion about others, baseless assumptions, and suspicion (zann). This is allowing presumption into one's heart without having any facts, which is especially harmful when one harbours a bad opinion about people who are outwardly righteous in appearance. Scholars have advised that one should even beware of forming conclusions based on the bad appearance of people, for it could be that God veiled their goodness from others. 

The Arabs traditionally were keen on having the ability to see a person's inner goodness. Once an Arab man came to the Prophet (SAW) to see who he was. When the man left, some men asked him about his opinion of the Prophet (SAW), and he said, "His face is not the face of a liar." This firāsah is, again, having the intuitive ability to see in people signs of goodness or evil. The Prophet (SAW) had this ability to the utmost degree and said that believers too possess it, but to a lesser extent. Sidi Ahmad Zarrūq said that every believer has the power of firāsah to varying degrees, based on the strength of their faith.

Backbiting of the Heart
However, having a bad opinion of someone without cause is considered a malady of the heart: "O you who believe, avoid suspicion, for some suspicion is sinful." - Quran, 49:12.  Often associated with this disease is backbiting (ghībah), which is speaking ill of another person behind their backs. It is possible to backbite in an unspoken form, which is when a person has unfounded negative thoughts against another. Suspicion in the heart that affects one's thoughts and opinion is considered backbiting of the heart (ghībat al-qalb). This is also not permissible. The Prophet (SAW) said, "Beware of a bad opinion, for it is the most false of speech." If someone says to you that a given person is bad, ask for proof. Without proof, it is tantamount to a lie. The sacred law of Islam (shariah) is based on proof and not conjecture. 

Imam Mawlūd says next that having doubts about someone's character is not forbidden if it is based on reason and observable evidence. This is different from suspicion that tends to be judgmental and, oftentimes, false. God says, "O you who believe, if an ungodly person brings you some news, then seek out its veracity." - Quran, 49:6. If someone known to openly indulge in major sins comes with some news, one should not accept it without caution. 

Sidi Ahmad Zarrūq gave good advice centuries ago that still remains relevant: "Do not trust anyone with matters related to your religion, your family, or your wealth until you have tested him at least a thousand times." According to this advice, one should test someone's sincerity and trustworthiness before entrusting him with anything significant. If there is much corruption in a given generation, it is best to be wary of people until their goodness becomes apparent. This is the advice of scholars.

Have a Good Opinion of God
Having doubts about people is different from decidedly judging them negatively. Forming a bad opinion in the presence of ample evidence is common sense. However, when people repent, they should not have their past held against them. If someone has sincerely repented, we should encourage them and make dua'a for them, and also support them in doing more good deeds. A hadith says, "There are two things no believer has been given anything better than: a good opinion of God and a good opinion of the servants of God."

According to sacred law, people are innocent until proven guilty. This relates to having a good opinion of God. A hadith sates that God says "I am in the opinion of My servant. If he thinks well of Me, he finds good, and if he thinks ill of Me, he finds evil." Another hadith states, "If [someone] finds good, let him thank God, and whoever finds other than that, let him blame only his own soul." The Prophet (SAW) also said that the affair of believers is all good, even if something unpleasant occurs, there is good in it.

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Source: Purification of the Heart by Hamza Yusuf