Deuteronomy 18:21 in the World English Bible
You may say in your heart, “How shall we know the word which the LORD has not spoken?”
Context
20 But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die.
21 And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?
22 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.