Deut 18:20-22 (NIV)
But a prophet who presumes to speak in my name
anything I have not commanded him to say, or a
prophet who speaks in the name of other gods, must
be put to death."
21 You may say to
yourselves, "How can we know when a message has
not been spoken by the LORD?" 22 If what a
prophet proclaims in the name of the LORD does
not take place or come true, that is a message the
LORD has not spoken. That prophet has spoken
presumptuously. Do not be afraid of him.