TT #190 · Parashas Shoftim · Fifth Aliyah · Passuk 18:22
That which the prophet speaks in the name of the Eternal and the thing does not happen and does not come – that is the thing the Eternal has not spoken; the prophet has spoken it presumptuously; you shall not be afraid of him.
Torah Temimah points to Sifrei Devarim 178 / Sanhedrin.
TT lemma: לֹא תָגוּר מִמֶּנּוּ.
Sefaria — Sifrei Devarim 178 / Sanhedrin
English: ‘You shall not be afraid of him’ – do not refrain from declaring him guilty.
Derash on the test of prophecy (Sifrei) — non-fulfillment is decisive proof against the claimant.
If a prophet’s word does not come to pass, he is exposed as false; one must not fear to act against him.