God says that we our words ought to be clean.

Ephesians 4: 29
Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers.

1 Peter 1: 15
But as he which hath called you is holy, so be ye holy in all manner of conversation;

2 Timothy 2: 16
shun profane and vain babblings: for they will increase unto more ungodliness.

Matthew 12: 36-37
...I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment. For by thy words thou shalt be justified, and by thy words thou shalt be condemned.