Each of these statements of Yeshua had the same core principle: What you think about becomes what you do.
Matthew 5:21-24 Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: (22) But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire. (23) Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; (24) Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
Matthew 5:27-28 Ye have heard that it was said by them of old time, Thou shalt not commit adultery: (28) But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Matthew 12:35 A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
Matthew 15:11 Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man.
(Hat tip to Doug W.)