Laws of Lashon Hara / 5; 3-4 (49)

3. There is a particular severity to saying about another that his not intelligent, as there can be no good intention to saying this. In addition, people tend to believe such statements more than they tend to believe other negative statements.

4. One who says about a Torah scholar that he is in fact not as learned as he appears to be, or likewise, about a person that he is not so knowledgeable in his profession, this is the epitome of lashon hara as this causes him hurt and damage.