Deuteronomy 23:21, 23
Deuteronomy 23:21 NIV
If you make a vow to the LORD your God, do not be slow to pay it, for the LORD your God will certainly demand it of you and you will be guilty of sin.
Deuteronomy 23:23 NIV
Whatever your lips utter you must be sure to do, because you made your vow freely to the LORD your God with your own mouth.