Deuteronomy 23:21 in the King James Version
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.
Context
20 You may charge a foreigner interest; but you shall not charge your brother interest, that the LORD your God may bless you in all that you put your hand to, in the land where you go in to possess it.
21 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.
22 But if you refrain from making a vow, it shall be no sin in you.