Mishnà su Proverbi 22:28

One who sells, the seller is permitted [to take the agricultural gifts] and the purchaser is forbidden. One may not hire a worker on the condition that the son [of the worker] should gather the gleanings after him. One who does not allow the poor to gather, or one who allows one but not another, or one who helps one of them [to gather] behold he is a robber of the poor. Concerning him it is said: “Do not remove the landmark of those that come up (olim)” (Proverbs 22:28).

What is peret? [Grapes] which fall down during the harvesting. If while he was harvesting [the grapes], he cut off an entire cluster by its stalk, and it got tangled up in the [grape] leaves, and then it fell from his hand to the ground and the single berries were separated, it belongs to the owner. One who places a basket under the vine when he is harvesting [the grapes], behold he is a robber of the poor. Concerning him it is said: “Do not remove the landmark of those that come up (olim)” (Proverbs 22:28).