Deuteronomy 18:21-22 (NKJV)

21 "And if you say in your heart, ‘How shall we know the word which the LORD has not spoken?’"
22 "when a prophet speaks in the name of the LORD, if the thing does not happen or come to pass, that is the thing which the LORD has not spoken; the prophet has spoken it presumptuously; you shall not be afraid of him."