Deuteronomy 23:21-23

When you make a vow to GOD, your God, don’t put off keeping it; GOD, your God, expects you to keep it and if you don’t you’re guilty. But if you don’t make a vow in the first place, there’s no sin. If you say you’re going to do something, do it. Keep the vow you willingly vowed to GOD, your God. You promised it, so do it.