Deuteronomy 18:20-22

20A false prophet may say that he speaks with my authority when that is not true. Or he may say that he has a message from another god. You must punish a false prophet with death.’

21You may say to yourselves, ‘How can we know when a message has not come from the Lord?’

22A prophet may say that he speaks with my authority, and he may say what will happen. But if it does not happen, you will know that his message has not come from me. He has not spoken my message. That prophet has spoken his own ideas. So do not be afraid of him!