A fasting heart is able to make the tongue fast with it, for “out of the abundance of the heart the mouth speaks.” (Matt 12:34).
The Lord also said, “But those things which proceed out of the mouth come from the heart, and they defile a man. “ (Matt 15:18). Moreover, “A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things.” (Matt 12:35). Therefore, if your heart abstains from sin, your tongue will abstain from every evil word.