כִּ֣י בְשֶׁ֔קֶר הֵ֛ם נִבְּאִ֥ים לָכֶ֖ם בִּשְׁמִ֑י לֹ֥א שְׁלַחְתִּ֖ים נְאֻם־יְהוָֽה׃ (ס)
Porque falsamente os profetizan ellos en mi nombre: no los envié, ha dicho SEÑOR.
Derashot HaRan
All of this, however, is contingent upon the condition that good is being prophesied, for a prophecy of evil can be nullified, as in the case of Ninveh. A prophecy of good, however, cannot be nullified, and this is the proof of our ultimate redemption. As Jeremiah said to Chananiah ben Azur (Jeremiah 29:9): "That prophet who prophesies for peace, when that prophesy materializes, it is known that the L-rd has sent that prophet in truth."