Deuteronomy 23:21-23 - Keeping Vows
21 When thou vowest a vow to Jehovah thy God, thou dost not delay to complete it; for Jehovah thy God doth certainly require it from thee, and it hath been in thee -- sin. 22 'And when thou forbearest to vow, it is not in thee a sin. 23 The produce of thy lips thou dost keep, and hast done it, as thou hast vowed to Jehovah thy God; a free-will-offering, which thou hast spoken with thy mouth.