EXTRA · Side-ring · Parashas Shoftim · Passuk 18:21
And if you say in your heart, ‘How shall we know the word that the Eternal has not spoken?’
Chazal / classical source (beyond TT-numbered main-ring units): Sifrei Devarim 178 / Rambam.
Lemma: וְכִי תֹאמַר בִּלְבָבֶךָ.
Sefaria — Sifrei Devarim 178 / Rambam
English: ‘And if you say in your heart’ – that you not remain silent from asking.
Derash legitimating the inquiry (Sifrei) — one is not only permitted but expected to seek a criterion.
It is proper for a person to ask in his heart how to know whether a claimed prophecy is from Hashem; the Torah provides the means of testing.