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.

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.