Deuteronomy 23
21 Ref When you shall vow a vow to Yahweh your God, you shall not be slack to pay it: for Yahweh 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 Ref That which is gone out of your lips you shall observe and do; according as you have vowed to Yahweh your God, a freewill-offering, which you have promised with your mouth.