Leviticus 7:16

If the sacrifice be a vow Offered in performance of a vow, the offerer having desired some special favour from God, and vowed the sacrifice to God if he would grant it. A voluntary offering, which a person offered freely to God, in testimony of his faith and love, not being under the obligation of any particular vow of his own, or command from God. On the morrow also the remainder shall be eaten Which was not allowed in the case of the thank-offering. The reason of which is to be fetched only from God's good pleasure and will, to which he expects our obedience, though we discern not the reason of his appointments.