Deuteronomy 23:21-23 - Keeping Vows
21 When thou hast vowed a vow unto the LORD thy God, see thou be not slack to pay it. For he will surely require it of thee, and it shall be sin unto thee. 22 If thou shalt leave vowing, it shall be no sin unto thee: 23 but that which is once gone out of thy lips, thou must keep and do, according as thou hast vowed unto the LORD thy God a freewill offering which thou hast spoken with thy mouth.