Midrash sobre Provérbios 22:28

2) "and the peret of your vineyard": (The taking of) peret obtains only with harvesting — whence they ruled: If he were harvesting, and in cutting the clusters, they became entangled in the leaves and fell to the ground and became peret (single grapes), these belong to the owner. If one left a basket under the vine while harvesting (so that any single grapes fall into it), he is robbing the poor (of their due). Of this it is written (Mishlei 22:28) "Do not remove the bound of yore."

And God appeared unto Jacob again, when he came from Padan-aram (Gen. 35:9). May it please our master to teach us: What is the punishment meted out to one who does not permit the poor to glean from his field? Thus do our masters teach us: One who does not allow the poor to glean from his field, or permits one and not another, or assists one of them at the time of reaping or harvesting, is considered guilty of robbing the poor. Concerning this it is said: Remove not the landmark of old ('olam) (Prov. 22:28). This should be read as “landmark of the poor” (olim).

Whence is it derived that if one removes the bounds of the tribes he transgresses a negative commandment? From (Proverbs 22:28) "Do not remove the long-standing boundary marker (which your forefathers have established.") Whence is it derived that if one confuses the words of R. Eliezer with those of R. Yehoshua, and vice versa, pronouncing clean, unclean, or unclean, clean — that he transgresses a negative commandment? From "You shall not move back the boundary marker of your neighbor." Whence is it derived that one who sells the burial plot of his fathers transgresses a negative commandment? From "You shall not move back the boundary marker of your neighbor." I might think that this were so even if no one were ever buried there; it is, therefore, written "in your inheritance that you shall inherit"

lo tefa'er": You shall not "lord it" over the poor man — whence they ruled: One who does not allow the poor to pick (the gleanings) or allows one but not another, or helps one of them (to pick) is robbing from the poor. And concerning such as this it is written (Proverbs 22:28) "Do not remove the bounds of yore."