Deuteronomy 18:20-22

20But the prophet who presumes to speak a message in my name that I have not commanded him to speak, or who speaks in the name of other gods — that prophet must die.’

21You may say to yourself, ‘How can we recognize a message the Lord has not spoken?’

22When a prophet speaks in the Lord’s name, and the message does not come true or is not fulfilled, that is a message the Lord has not spoken. The prophet has spoken it presumptuously. Do not be afraid of him.