וְכִ֥י תֹאמַ֖ר בִּלְבָבֶ֑ךָ אֵיכָה֙ נֵדַ֣ע אֶת־הַדָּבָ֔ר אֲשֶׁ֥ר לֹא־דִבְּר֖וֹ יְהוָֽה׃
And if thou say in thy heart: ‘How shall we know the word which the LORD hath not spoken?’
Read midrashic interpretation of Deuteronomy 18:21: and classical rabbinic exegesis.