Surah 4. An-Nisaa', Ayah 148

As some of the commentators (e.g., Razi) point out, this may refer to giving currency to earlier sayings or deeds of the repentant sinners - both hypocrites and outright deniers of the truth - mentioned in the preceding two verses: an interpretation which seems to be borne out by the context. However, the above statement has a general import as well: it prohibits the public mention of anybody's evil deeds or sayings, "unless it be by him who has been wronged [thereby]" - which also implies that evil behaviour which affects the society as a whole may be made public if the interests of the wronged party - in this case, the society as such - demand it.

 i.e., Allah does not like talking about people behind their backs—except when someone who has been wronged seeks counselling or assistance from authorities.

We can make a public scandal of evil in many ways. (1) It may be idle sensation-mongering: it often leads to more evil by imitation, as where criminal deeds are glorified in a cinema, or talked about shamelessly in a novel or drama. (2) It may be malicious gossip of a foolish, personal kind: it does no good, but it hurts people's feelings. (3) It may be malevolent slander or libel: it is intended deliberately to cause harm to people's reputation or injure them in other ways, and is rightly punishable under all laws. (4) It may be a public rebuke or correction or remonstrance, without malice. (1), (2) and (3) are absolutely forbidden. (4) may be by a person in authority; in which case the exception applies, for all wrong or injustice must be corrected openly, to prevent its recurrence. Or (4) may be a person not vested with authority, but acting either from motives of public spirit, or in order to help some one who has been wronged; here again the exception will apply. But if the motive is different, the exception does not apply. (4) would also include a public complaint by a person who has suffered a wrong; he has every right to seek public redress.