Leviticus 23:38
Beside the sabbaths of the LORD, and beside your gifts, and beside all your vows, and beside all your freewill offerings, which you give to the LORD.
Torrey's Topical Textbook—AKJV

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:
Torrey's Topical Textbook—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.
Torrey's Topical Textbook—AKJV