Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 214
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 214, with English translation by Maimonides (Rambam).
שלא ליקח עומר השכחה, שנאמר "לא תשוב לקחתו" (דברים כד,יט); וכן לכל האילנות יש שכחה, שנאמר "לא תפאר אחריך" (דברים כד,כ).
Not to take a sheaf which has been forgotten, as [Deuteronomy 24:19] states: "Do not go back to take it." [This prohibition also applies to] all trees, as [ibid. 24:20] states: "Do not carefully re-harvest it."