(Devarim 18:20) "then he shall be put to death": by the unqualified "death penalty" of the Torah, i.e., strangulation.
(Ibid. 21) "And when you say in your heart: How shall we know the thing which the L-rd has not spoken?": Jeremiah said about the remaining (Temple) vessels (Jeremiah 27:22) "They shall be brought to Bavel and there shall they remain." Chananiah (ben Azur) said (Ibid. 28:3) "I will restore to this place all the vessels of the house of the L-rd." Whom shall we heed? It is, therefore, written (Devarim, Ibid. 22) "What the prophet shall speak in the name of the L-rd and the thing not be and not come to pass, that is the thing which the L-rd has not spoken." "In presumption has the prophet spoken it.": He is liable (to the death penalty) for (willful) presumption, and not for unwittingness.
"Do not hold back from him": Do not hold yourself back from incriminating him.