Deuteronomy 23:21-23

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