Deuteronomy 23:21 - 23:23
21 “When you make a vow to Yahweh your God, you shall not delay to pay it, for Yahweh your God will surely require it of you; and it will be a sin in you. 22 However, if you refrain from vowing, it will not be a sin in you. 23 You shall be careful and do what goes out from your lips, just as you have voluntarily vowed to Yahweh your God that which you spoke with your mouth.