Numbers 30:3

נדר A VOW — This is when one says, Behold, I take upon myself an obligation which is as sacred to me as an offering, that I will not eat, or that I will not do such-and-such a thing. — One might think that if he swears that he will eat the flesh of an animal which has not been slaughtered properly according to the rite that I may apply to him the text, “He shall do according to all that goeth forth from his mouth”! It, however, says, “[If he takes an oath] to forbid something to himself”, i.e., to forbid for himself something which is permissible to him — and not to make permissible that which is forbidden to him (Sifrei Bamidbar 153:4). לא יחל דברו This is the same as לא יחלל דברו (the root of יחל being חלל): He shall not make his own word to bear a secular character (חולין) (Sifrei Bamidbar 153:4; see also Rashi on Leviticus 21:4).

אִישׁ֩ כִּֽי־יִדֹּ֨ר נֶ֜דֶר לַֽיהֹוָ֗ה אֽוֹ־הִשָּׁ֤בַע שְׁבֻעָה֙ לֶאְסֹ֤ר אִסָּר֙ עַל־נַפְשׁ֔וֹ לֹ֥א יַחֵ֖ל דְּבָר֑וֹ כְּכׇל־הַיֹּצֵ֥א מִפִּ֖יו יַעֲשֶֽׂה׃

If a man vow a vow to the Lord, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceeds out of his mouth.