Negative commandment · Lo Ta’aseh #214 of 613
Not to take a sheaf which has been forgotten
שלא ליקח עומר השכחה
The verse · Devarim 24:19
כִּי תִקְצֹר קְצִירְךָ בְשָׂדֶךָ וְשָׁכַחְתָּ עֹמֶר בַּשָּׂדֶה לֹא תָשׁוּב לְקַחְתּוֹ לַגֵּר לַיָּתוֹם וְלָאַלְמָנָה יִהְיֶה לְמַעַן יְבָרֶכְךָ ה׳ אֱלֹהֶיךָ בְּכֹל מַעֲשֵׂה יָדֶיךָ
“When thou reapest thy harvest in thy field, and hast forgotten a sheaf in the field, thou shalt not go back to fetch it: it shall be for the stranger, for the fatherless, and for the widow: that the Lord thy God may bless thee in all the work of thy hands.”
The Rambam’s words
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."
שלא ליקח עומר השכחה, שנאמר "לא תשוב לקחתו" (דברים כד,יט); וכן לכל האילנות יש שכחה, שנאמר "לא תפאר אחריך" (דברים כד,כ).
Why · Sefer HaChinuch #593
To not go back to take what is forgotten
“It is from the roots of the commandment [that it is] since the poor and destitute — in their sin and in their poverty — suspend their eyes upon the produce, in their seeing the owners of the field sheaving their sheaves “according to the blessing of the Lord that He gave to them”; and they think in their heart, saying, “If only it will be like this for me, to gather sheaves into my house — and if only I could bring one, I would rejoice in it”; and hence it was from His kindnesses towards His creatures, may He be blessed, to fulfill this desire of theirs when it occurs that the owner of the field forgets it.”
Where in the Mishneh Torah
Hilchot Gifts to the Poor הלכות מתנות עניים