Rashi on Deuteronomy 24:19/רש"י על דברים כ״ד:י״ט

1ושכחת עמר [WHEN THOU REAPEST THY HARVEST IN THY FIELD] AND HAST FORGOT A SHEAF … [THOU SHALT NOT RETURN TO TAKE IT] — a sheaf, but not a stack of corn (Sifrei Devarim 283:1). Hence they (the Rabbis) said, “A sheaf that contains two Seahs of grain which one has forgotten in the field, does not come under the term of שכחה (a forgotten sheaf)” (Mishnah Peah 6:6).

2בשדה [AND HAST FORGOT …] IN THE FIELD — This apparently redundant word (it has just stated בשדך) is intended to include under this law forgotten “standing corn” (קמה) — that one has forgotten to cut down part of it (Sifrei Devarim 283:2).

3לא תשוב לקחתו THOU SHALT NOT GO BACK TO TAKE IT — Hence they (the Rabbis) said: A sheaf that is behind him is considered as שכחה, that which is in front of him is not considered שכחה, for the expression “thou shalt not go back [to take it]” is not applicable to it (Mishnah Peah 6:4; Bava Metzia 11a).

4למען יברכך THAT [THE LORD THY GOD] BLESS THEE — although it came into his (the stranger’s or the orphan’s) hand without him (the owner) intending this; it follows logically that he will certainly receive a blessing if he does this intentionally! — You must therefore admit that if a Sela fell out of one’s hand and a poor man found it and supports himself by it, then he will surely be blessed on that account (Sifrei Devarim 283:6; cf. Rashi on Leviticus 5:17 end).