Deuteronomy 16:10
And you shall keep the feast of weeks to the LORD your God with a tribute of a freewill offering of your hand, which you shall give to the LORD your God, according as the LORD your God has blessed you:
Nave's Topical Index—AKJV

Deuteronomy 23:23
That which is gone out of your lips you shall keep and perform; even a freewill offering, according as you have vowed to the LORD your God, which you have promised with your mouth.
Nave's Topical Index—AKJV