Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 33

33. But, like as it is asked of what mouth the Scripture saith, “The mouth that lieth, slayeth 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 profiteth 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 doth utter what he forgeth against any, but even without speaking wisheth him to be thought such; which is in truth to detract with the mouth of the heart; which thing, it saith, cannot be obscure and hidden from God.