Deuteronomy 23:21 in the World English Bible
When you vow a vow to the LORD your God, you shall not be slack to pay it, for the LORD your God will surely require it of you; and it would be sin in you.
Context
20 Unto a stranger thou mayest lend upon usury; but unto thy brother thou shalt not lend upon usury: that the LORD thy God may bless thee in all that thou settest thine hand to in the land whither thou goest to possess it.
21 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.
22 But if thou shalt forbear to vow, it shall be no sin in thee.