There is a concept that while a positive prophecy must come to pass (at least when it is a national prophecy) a negative one can be retracted if the subjects to teshuvah (repent)? While there a number of places where this idea is found, I would like to know if anyone can identify a particular posuk (verse) in Sefer Yirmeyahu where I once saw this idea stated explicitly? (This isn't rhetorical, can't remember where it is anymore).
ח הַנְּבִיאִים, אֲשֶׁר הָיוּ לְפָנַי וּלְפָנֶיךָ--מִן-הָעוֹלָם: וַיִּנָּבְאוּ אֶל-אֲרָצוֹת רַבּוֹת, וְעַל-מַמְלָכוֹת גְּדֹלוֹת, לְמִלְחָמָה, וּלְרָעָה וּלְדָבֶר. ט הַנָּבִיא, אֲשֶׁר יִנָּבֵא לְשָׁלוֹם--בְּבֹא, דְּבַר הַנָּבִיא, יִוָּדַע הַנָּבִיא, אֲשֶׁר-שְׁלָחוֹ יְהוָה בֶּאֱמֶת. י
And see context in the preceding above verses. Paraphrasing slightly: a different prophet just said "everything will be hunky-dory", to which Jeremiah replies "Amen, I hope you're right! ... May prophets including myself have prophesied calamities, only some of which have happened; only a prophecy for the good is carved in stone."
(Chief Rabbi Sacks discussed this in Shoftim 5769. He quotes Rambam's formulation of the concept in Chapter 10 of Yesodei HaTorah; hat tip to Machon Mamre for hyperlinking the citation to Jeremiah.)