Deuteronomy 18:21-22

Young's Literal Translation

²¹ `And when thou sayest in thy heart, How do we know the word which Jehovah hath not spoken? --

²² that which the prophet speaketh in the name of Jehovah, and the thing is not, and cometh not -- it [is] the word which Jehovah hath not spoken; in presumption hath the prophet spoken it; -- thou art not afraid of him.