Deuteronomy 23:21-23 - Keeping Vows
21 When thou shalt vow a vow to Jehovah thy God thou shalt not delay to complete it, for Jehovah thy God requiring, will require it from thee; and it was sin in thee. 22 And if thou shalt cease to vow, it shall not be sin in thee. 23 That going out of thy lips, thou shalt watch and do according as thou didst vow to Jehovah thy God; a voluntary gift which thou spakest with thy mouth.