When a prophet speaks in the name of the LORD and the message does not come to pass or come true, that is a message the LORD has not spoken. The prophet has spoken presumptuously. Do not be afraid of him.
KJV
When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that [is] the thing which the LORD hath not spoken, [but] the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
ASV
when a prophet speaketh in the name of Jehovah, if the thing follow not, nor come to pass, that is the thing which Jehovah hath not spoken: the prophet hath spoken it presumptuously, thou shalt not be afraid of him.