Shemiras Halashon Halachos

Hilchos Lashon Hara chapter 10, section 15

We see that when relying on this leniency a person is in great danger of speaking lashon hara if he does not observe all the details involved, especially nos. 2 and 4. Regarding this, Life and death are in the hand of the tongue (Proverbs 18:21). If a person does not think carefully before speaking he will certainly stumble, because as he talks he becomes overcome with anger and a person cannot guard against this. Therefore, one must plan what to say before speaking.

Hilchos Lashon Hara chapter 10, section 16

All this indicates what a mistake people make. If one asks someone why he spoke lashon hara about someone else he replies, "Because he spoke about me to so and so." This is a mistake. First, if one heard of his speaking about oneself from the person he spoke to, one may not believe him because it forbidden to believe lashon hara. Also, even if it is true, it is still forbidden to speak about him as we said in paragraph 11.