Mistrust is when your heart judges a person to be bad, based on rumours, and suspicions. Whenever a person finds this in himself he should expel those thoughts, for judging a person should only be done with evidence like seeing the sun. As for whispers which do not settle in the heart, then they are pardoned, and it is upon people when they get these whispers to seek refuge with Allah from the accursed devil.
It was stated in the book “Ihyaa’ Uloom Al deen”:-
I know that thinking badly of someone is forbidden, like saying bad things, just as it is forbidden for you to speak to others with your own tongue about the misdeeds of others. Therefore, you are not allowed to think ill of your brother, and I do not mean by it anything apart from the matters of the heart and judging others to be bad. As for thoughts and the talk of your conscious, it is forgiven, and doubts are also forgiven, but it is forbidden to suspect.
Suspicion is what the soul is left to, and what the heart is inclined to, for Allah SWA says: “O you who believe! avoid most of suspicion, for surely suspicion in some cases is a sin”. The reason for it being forbidden is that the secrets of the heart are only known by the One who knows the unseen, it is not for you to assume ill of others unless it is revealed to you by an eye that does not require interpretation, and when this happens you can only believe in what you witnessed and saw. As for what you have not seen with your eyes, or heard with your ears then it enters your heart, the Satan has thrown it at you, then you shouldn’t listen to him because he is the most immoral, and Allah has said: (O you who believe! if an evil-doer comes to you with a report, look carefully into it, lest you harm a people in ignorance) for it is not permissible to believe what the devil has whispered to you.
And if you say: How is it known that you have a suspicion or doubts that the soul speaks? then we say: the sign of thinking ill of someone is that the heart changes the way it feels about that person so that it stays away from him, makes him disliked, and it weighs him down, takes his attention and does not honor him, and is oblivious to him – these are the signs and realities of having suspicion.
The prophet ﷺ said: “There are three things in the believer and he has a way out of them, so his exit is from a bad thought that he does not actualize it”. This means that he does not achieve it in himself with action, in the heart or in the limbs. As for the heart, it changes to resentment and hatred. As for the limbs, it is to act according to it. Devil may decide on the heart, with the slightest imagination, to harm people, and tell him that this is from your intelligence, speed of understanding, or that he is believer and the believer looks at things through the light of Allah, when he is actually obsessed with the deception and darkness of Satan.