Deuteronomy 23:21
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.
World English Bible (Public Domain)
In context
Before · Deuteronomy 23:19–20
19You shall not lend on interest to your brother: interest of money, interest of food, interest of anything that is lent on interest. 20You 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.
21When 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.