العدد 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).

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.

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