What is the authenticity of the hadith: "There is no backbiting for a transgressor," and is warning against someone who has the evil eye considered backbiting him? And who are those about whom it is permissible to issue a warning without that being considered backbiting or slander?

Backbiting is forbidden by Islamic law, but it is permissible in limited instances for an overriding benefit, such as for one who has been wronged, one who is identifying someone, one who is warning, one who openly commits sins, one who is seeking a fatwa, and one who seeks help in removing an evil. As for the phrase "there is no backbiting for a transgressor," it is not a hadith. However, the legal proofs indicate the permissibility of backbiting a transgressor who openly commits sins and disobedience, because he has no sanctity in that regard.