Leviticus 25:21

For three years — Not compleatly, but in great part, namely, for that part of the 6th year which was between the beginning of harvest and the beginning of the 7th year, for the whole 7th year, and for that part of the 8th year which was before the harvest, which reached almost until the beginning of the ninth year. This is added to shew the equity of this command. As God would hereby try their faith and obedience, so he gave them an eminent proof of his own exact providence and tender care over them in making provisions suitable to their necessities.