Deuteronomy 23:21-23

WEB(i) 21 When you 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 refrain from making a vow, it shall be no sin in you. 23 You shall observe and do that which has gone out of your lips. Whatever you have vowed to Yahweh your God as a free will offering, which you have promised with your mouth, you must do.