Halakhah sa Jonas 3:5: Mishneh Torah, Foundations

Concerning a prophet's prediction of calamities, such one would die, or such year would be visited by famine, or war, or matters similar to these, even though his words be not established, it is not considered contradictory to his prophecy. It shall not be said: "Behold, he spoke and it did not come to pass!" For, the Holy One, blessed is He! is long-suffering and abundant in goodness, and repenteth Him of the evil, and it is possible that they repented and were forgiven, as did the inhabitants of Nineveh,²Jonah, 3. 5-8 G. or that their fate was postponed, as was with Hezekiah.³Isa. 38; 11 Kings, 20.6 G. But if he assured that good would come to pass, saying that it would be thus and such, but the good he promised did not come to pass, it is certain that he is a false prophet, for every good thing God decides upon, even though it be contingent, He doth not repent. And, we find no instance when He repented from a good thing save only at the destruction of the First Temple, when he assured the righteous that they would not die together with the wicked and retracted it, as it appears in Tractat Shabbat⁴Shabbat, 55a. Because the wickedness was great, and the righteous did not sufficiently rebuke the evildoers. See Shabbat, 119b. G.. Herefrom we learn that a prophet is tested solely by his prophecy concerning good tidings, which is in harmony with what Jeremiah said in his reply to Hananiah son of Azzur, when Jeremiah was prophesying evil tidings and Hananiah good, saying unto him: "If my words will not be established, it would yet not be evidence that I am a false prophet; but if thy words be not established it will be proclaimed that thou art a false prophet". For it is said: "Nevertheless hear thou now this word that I speak in thine ears, and in the ears of all the people: The prophets that have been before me and before thee of old prophesied against many countries, and against many kingdoms, of war, and of evil, and of pestilence; the prophet that prophesieth of peace, when the word of the prophet shall come to pass, then shall the prophet be known, that the Lord hath truly sent him" (Jer. 28.7–9).