Deuteronomy 23:21-23 -

A vow to the Lord, once made, was to be religiously kept; the Lord would require it, and to refuse or neglect to pay it would be held a sin. No one, however, was under any obligation to vow—that was to be a purely voluntary act. That which is gone out of thy lips thou shalt keep and perform … according as thou hast vowed unto the Lord thy God of free-will ( נְדָבָה , spontaneously). (For the law concerning vows in general, see Leviticus 27:1-34 , and Numbers 30:1-16 .)