“Watch your mouth!” Did your parents ever have to say that to you? Maybe you said something hurtful or sinful. In the 9th commandment, God says: “You shall not give false testimony against your neighbor.” (Exodus 20:16). “False testimony” is not just for the court of law; it is saying something untrue about others. Jesus, for example, was said to be in league with Satan. Jesus uses the situation to talk about speech. As with other commandments, Jesus gets at the attitude as well as the action, the heart as well as the mouth. 
Scripture: Exodus 20:16; Matthew 12:22-37