Deuteronomy 18:21
King James Version
“And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?”
Context
18I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
19And it shall come to pass, whosoever will not hearken unto my words which he shall speak in my name, I will require of him.
20But 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.
21And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?
22When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that the thing which the LORD hath not spoken, the prophet hath spoken it presumptuously: thou shalt not be afraid of him.