And if you say in your heart, "How shall we know the word which the LORD has not spoken?'--
Deuteronomy 18:21
Surrounding context
18:20 But the prophet who presumes to speak a word in My name, which I have not commanded him to speak, or who speaks in the name of other gods, that prophet shall die.'
18:21 And if you say in your heart, "How shall we know the word which the LORD has not spoken?'--
18:22 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.