Hafetz Haim Halachot
Halacha 15 / 215

Hilchos Lashon Hara, Opening – negative commands (7)

Sometimes the speaker also breaks the negative commandment (Vayikra 19:17), "Do not hate your fellow in your heart." This happens when he speaks nicely to a person's face and speaks badly about him to others. And if he clearly tells the listeners not to go and tell that person, he surely breaks this negative commandment.