Deuteronomy 23:21-23 - Keeping Vows
21 When you make a vow to the Lord your God you must not delay in fulfilling it, for otherwise he will surely hold you accountable as a sinner. 22 If you refrain from making a vow, it will not be sinful. 23 Whatever you vow, you must be careful to do what you have promised, such as what you have vowed to the Lord your God as a freewill offering.