Numbers 30:1

This law was delivered to the heads of the tribes that they might instruct those who were under their charge, explain the law to them, give then necessary cautions, and call them to account, if there were occasion, for the breach of their vows. Perhaps the heads of the tribes had, upon some emergency of this kind, consulted Moses, and desired by him to know the mind of God, and here they are told it: This is the thing which the Lord has commanded concerning vows, and it is a command still in force. 1. The case supposed is that a person vows a vow unto the Lord, making God a party to the promise, and designing his honour and glory in it. The matter of the vow is supposed to be something lawful: no man can be by his own promise bound to do that which he is already by the divine precept prohibited from doing. Yet it is supposed to be something which, in such and such measures and degrees, was not a necessary duty antecedent to the vow. A person might vow to bring such and such sacrifices at certain times, to give such and such a sum or such a proportion in alms, to forbear such meats and drinks which the law allowed, to fast and afflict the soul (which is specified Num 30:13) at other times besides the day of atonement.

— Matthew Henry's Concise Commentary (public domain)