Deuteronomy 23:23-25

But once you have voluntarily made a vow, be careful to fulfill your promise to the LORD your God. “When you enter your neighbor’s vineyard, you may eat your fill of grapes, but you must not carry any away in a basket. And when you enter your neighbor’s field of grain, you may pluck the heads of grain with your hand, but you must not harvest it with a sickle.