Deuteronomy 18:21

And if thou shalt say in your thoughts, How shall we know the word which the Lord hath not spoken?

V’CHI'(AND WHEN) THOU SAY IN THY HEART: ‘HOW SHALL WE KNOW THE WORD WHICH THE ETERNAL HATH NOT SPOKEN?’ “You are destined to say this when Hananiah the son of Azur [the false prophet] will come and prophesy, Behold, the vessels of the Eternal’s House shall now shortly be brought back from Babylon while Jeremiah [the true prophet] stands and proclaims concerning the pillars, and concerning the sea, and concerning the bases, and concerning the residue of the vessels, Verse 19. which were not exiled with Jeconiah, they shall be carried to Babylon, Verse 22. with the exile of Zedekiah.” This is Rashi’s language from the Sifre. 178. This interpretation [of the Sifre] is based on the fact that the verse does not state “v’im (and if) thou say in thy heart” [but it states ‘v’chi’ — (and when) thou say in thy heart]. This is as I have mentioned that the Torah tells future events through allusions. But the purport of the verse is to say, “How shall we know the word which the Eternal hath not spoken, so that we can slay the one who so prophesies?” And he commanded that if the event does not come to pass then he should be killed because the Eternal hath not spoken it. Do not fear him because of his wisdom or because of the sign that he gave [even though] the sign or the wonder did come to pass. Similarly the people could ask, “How shall we know the word which G-d had spoken, to which we have been obligated to hearken in whatever He shall command?” But this [Verse 22] is the answer to both questions, for, in the case of every prophet we should look ahead to that which he foretold. If it does not come to pass we shall know that he is a false prophet and he is to be killed, and if all that he predicted does come to pass he is established to be a prophet of the Eternal, and we are bound to hearken to him in all that he commands in the name of G-d, just as he said, unto him ye shall hearken. Even to transgress the words of the Torah, [we are required to obey him] provided that it is to meet the particular need of the time, as in the case of Elijah on Mount Carmel [who, in his attempt to discredit idolatry, brought offerings to G-d on Mount Carmel despite the fact that it was forbidden to do so outside the Sanctuary Court in Jerusalem]. This is the sense of the expression, [a prophet] like unto me, [And G-d said, I will raise them up a prophet] like unto …