Deuteronomy 23:21-23

21When thou shalt vow a vow unto Yhwh thy God, thou shalt not slack to pay it: for Yhwh thy God will surely require it of thee; and it would be sin in thee. 22But if thou shalt forbear to vow, it shall be no sin in thee. 23That which is gone out of thy lips thou shalt keep and perform; even a freewill offering, according as thou hast vowed unto Yhwh thy God, which thou hast promised with thy mouth.