Deuteronomy 23:21, 23

When thou shalt vow a vow unto the LORD thy God, thou shalt not slack to pay it: for the LORD thy God will surely require it of thee; and it would be sin in thee.

That which is gone out of thy lips thou shalt keep and perform; even a freewill offering, according as thou hast vowed unto the LORD thy God, which thou hast promised with thy mouth.