Having bad opinions based on only Suspicion (zaan) about a good person is not allowed. And this is linked to Ghibat'l Qalb – Backbiting of the Heart.
The Quran tells us ‘O you who believe! avoid most of suspicion, for surely suspicion in some cases is a sinful, and do not spy nor let some of you backbite others. Does one of you like to eat the flesh of his dead brother? But you abhor it; and be careful of (your duty to) Allah, surely Allah is Oft-returning (to mercy), Merciful. 49.12
The Key element here is about proof, if someone has proof about a person then produce your evidence before giving claims about a persons name and honour.
We are ordered to avoid suspicion in most cases. This signifies that at times suspicion may be in order. We read in the same surah the following order:
"Believers, if an evil-doer brings you a piece of news, inquire first into its truth, lest you should wrong others unwittingly and then regret your action." (49;6)
Hence, when we hear information about other people, we must not jump to conclusions. We must first make sure of the truth of what we have heard. The information is kept in the balance until it is proven to be true.
Tradition "Be careful of suspicion, for it is the most mistaken of all speech. Do not spy on others, compete * among yourselves, envy one another, or despise one another. Rather, be servants of Allah and brothers!" There is nothing wrong with healthy competition, particularly in doing good deeds. This hadith refers to competition that destroys team play and equality, etc.
Tradition "Sometimes a victim of robbery will become so suspicious [of everyone around him] that he will become worse than the thief." In other words, his suspicions will lead him to doubt everyone until he has destroyed every one of his relationships
It is permissible to call over a person and inform them of the facts, so that they will not be suspicious, as we see in the following hadith (narration).
Tradition a man passed by while the Prophet of Allah, upon him be peace, was with one of his wives. The Prophet called the man and said, 'O So and so. This is my wife, so and so." The man replied, What I might have thought about another, I would never think about you." The Prophet, upon him be peace, said, "Shaytan [satan] will circulate through a person just like blood circulates [through one's veins]."