Deuteronomy 23:21-23 - Keeping Vows
21 When you shall 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 shall forbear to vow, it shall be no sin in you. 23 That which is gone out of your lips you shall observe and do; according as you have vowed to the LORD your God, a freewill offering, which you have promised with your mouth.