When you have a bad suspicion about a Muslim in your heart, you should increase your consideration toward him and make dua for him. This way you will infuriate Satan and will be able to drive him away. Because of this, Satan will be reluctant to put evil thoughts into your heart, afraid that you would increase your consideration towards your brother and will make dua for him again.
Understand that for every rule which I have mentioned from the Quran, the Devil has one to match it, which he puts beside the proper rule to cause error.
