Discourse on Defilement
"Listen and understand. What goes into a man's mouth does not make him 'unclean,' but what comes out of his mouth, that is what makes him 'unclean.'".
The Gospel of Mark has a similar episode in which Jesus explains how a man is defiled by evil that comes out of him:
"What comes out of a man is what makes him 'unclean.' For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. All these evils come from inside and make a man 'unclean.'"