Deuteronomy 23:21-23

21If 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. 22But if you refrain from making a vow, you will not be guilty.

23Whatever your lips utter you must be sure to do, because you made your vow freely to the LORD your God with your own mouth.