Musar sur Rois 1 1:26

“And not be like Korach and like his community” (Numbers 17:5). And our Rabbis, may their memory be blessed, said (Sanhedrin 110a), “Anyone who perpetuates a dispute violates a negative commandment, as it is stated ‘and not be like Korach and like his community.’” But it is permissible to say evil speech about disputatious people, as it is stated (I Kings 1:26), “But he did not invite me your servant, or the priest Zadok, etc.”