The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
Luke 6:45 ESV
Jesus says here that we speak out of the fullness, or true essence, of our hearts. What comes out is a reflection of what is already in. Our speech displays what we think about, read about, pray about, etc. Clean words flow from a clean heart and mind.