Rashi on Numbers 30:3/רש"י על במדבר ל׳:ג׳

1נדר 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).

2לא 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).