Deuteronomy 16:10

And thou shalt keep the Feast of Weeks, or Pentecost, unto the Lord, thy God, with a tribute, or measure, of a free-will offering of thine hand, which thou shalt give unto the lord, thy god, according as the Lord, thy God, hath blessed thee, the offering thus being as rich as the worshiper desired, in proportion to the blessing which he had received at the Lord's hand. Exodus 23:15; Exodus 34:20;