Deuteronomy 23:21-23 - Keeping Vows
21 When thou shall vow a vow to LORD thy God, thou shall not be slack to pay it. For LORD thy God will surely require it of thee, and it would be sin in thee. 22 But if thou shall forbear to vow, it shall be no sin in thee. 23 That which has gone out of thy lips thou shall observe and do, according as thou have vowed to LORD thy God, a freewill-offering, which thou have promised with thy mouth.