In The Name of Allaah, The Most Merciful The Bestower of Mercy
Allaah (The Most High) said: ”O you who believe! Avoid much suspicions, indeed some suspicions are sins”. 49:12
Imaam as-Saa’di (rahimahullaah) said: Allaah (The Most High) has forbidden much evil (sinful) suspicion towards the believers, because some suspicions are sins. And that is like the suspicion devoid of reality and factual evidence, and like the suspicion accompanied by much forbidden speech and action. For indeed, the settlement of evil suspicion in the heart does not restrict the one (in whose heart is that evil suspicion) only to that; rather, he does not cease until he utters and does that which is not permissible/befitting.
[Source: Tafseer As-Sadi]