Laws of Guarding One's Tongue
Hilchos Lashon Hara, Opening – negative commands (12)
If a person engenders controversy through his story, he transgresses the negative command (Bamidbra 17:5), "You shall not be like Korach and his congregation," which is a prohibition against engendering controversy.
Hilchos Lashon Hara, Opening – negative commands (13)
Often, an additional negative command is incurred when people often shame a person for things he did in the past, such as having a lowly family pedigree, lack of intelligence, or lack of success, each person according to what upsets him. Even if this happened with only the two of them present, one transgresses the negative command, "A man shall not afflict his fellow," and how much more so if this happened before [other] people.