משנה אויף דברים 24:19

These are the things that have no definite quantity: The corners [of the field]. First-fruits; [The offerings brought] on appearing [at the Temple on the three pilgrimage festivals]. The performance of righteous deeds; And the study of the torah. The following are the things for which a man enjoys the fruits in this world while the principal remains for him in the world to come: Honoring one’s father and mother; The performance of righteous deeds; And the making of peace between a person and his friend; And the study of the torah is equal to them all.

Peah is given from [the crop] while it is still connected with the soil. But in the case of hanging vine-branches and the date-palm, the owner brings down [the fruit] and distributes it among the poor. Rabbi Shimon says: the same applies to smooth nut trees. Even if ninety-nine [of the poor] say [to the owner] to distribute it and one says to leave it in the field, this latter is listened to, since he spoke in accordance with the halakhah.

These are to be considered ends of the rows:If two men begin [to gather] from the middle of the row, one facing north and the other south and they forget [some sheaves] in front of them and behind them, those left in front of them are “forgotten,” but those left behind them are not “forgotten.” If an individual begins from the end of the row and he forgets [some sheaves] in front of him and behind him, those in front of him are not “forgotten”, whereas those behind him are “forgotten,” for this comes under the category of “you shall not go back [to retrieve it].” This is the general rule: anything that can be said to fall under the law “you shall not go back” is considered “forgotten,” but that to which the principle of “you shall not go back” cannot be applied is not considered “forgotten.”