Matthew 7:16-20 (Also found in Luke 6:43-44)
You shall know them by their fruits.
Do men gather grapes of thorns or figs of thistles?
Every good tree bringeth forth good fruit; but a corrupt tree brings forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that does not bring forth good fruit is cut down, and cast into the fire.
Therefore by their fruits you shall know them.
Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
O generation of vipers, how can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks. A good man out of the good treasure of the heart brings forth good things: and an evil man out of the evil treasure brings forth evil things.
But I say to you, That every idle word that men speak, they shall give account thereof in the day of judgment. For by your words you shall be justified, and by your words you shall be condemned.