Proverbs 12:14

A man will be satisfied with good from the fruit of his words, and the deed of a man’s hands will return to him (as a harvest).

What we believe in our hearts, we eventually speak.  The bible tells us to monitor our thoughts and not allow thoughts contrary to the word of God to form.  Why! Because we will create with our words what we imagine in our minds.  When we keep our minds clear of the negative, we will only speak the life-affirming words of God.