Nachschlagewerk zu Dewarim 13:4

אשר ידבר הנביא WHEN THE PROPHET SPEAKETH [IN THE NAME OF THE LORD] and says, “This thing will once happen to you”, and you will see that it did not happen, הוא הדבר אשר לא דברו ה׳ THAT IS THE THING WHICH THE LORD HATH NOT SPOKEN — and, therefore, slay him. If, however, you say, “But this refers to one who will prophesy about the future. But suppose one comes and says, ‘Do so and so, and I say this by the command of the Holy One blessed be He’? How, then, can we know whether God has spoken this or not?” The reply is: As regards such a case they have already been commanded that if one comes to thrust thee away from one of the divine commandments, לא תשמע לו “then thou shalt not hearken unto him” (Deuteronomy 13:12), unless you are certain that he is a perfectly righteous man, as, for instance. Elijah at the incident on Mount Carmel, who offered sacrifices on a Bamah (an improvised altar) at a time when offering on Bamoth was forbidden, but who did so in order to fence Israel in against idolatry. Thus all depends on the needs of the time and the necessity of taking preventive measures (סיג) against a breach. With reference to such a case it states, (v. 15) “unto him ye shall hearken” (Sanhedrin 89b; Yevamot 90b).