33 But, like as it is asked of what mouth the Scripture says, “The mouth that lies, slays the soul,” so it may be asked, of what lie. For it seems to speak of that lie in particular, which consists in detraction. It says, “Keep yourselves from murmuring, which profits nothing, and from detraction refrain your tongue.” Now this detraction takes place through malevolence, when any man not only with mouth and voice of the body does utter what he forges against any, but even without speaking wishes him to be thought such; which is in truth to detract with the mouth of the heart; which thing, it says, cannot be obscure and hidden from God.
Source: On Lying (New Advent)