Deuteronomy 23:21-23 - Keeping Vows
21 When thou shalt vow a vow to the LORD thy God, thou shalt not defer to pay it: for the LORD thy God will surely require it of thee; and it would be sin in thee. 22 But if thou shalt forbear to vow, it shall be no sin in thee. 23 That which is uttered by thy lips thou shalt keep and perform; even a free-will-offering, according as thou hast vowed to the LORD thy God, which thou hast promised with thy mouth.