Positive commandment · Mitzvat Aseh #122 of 613
To leave a forgotten sheaf
להניח עומר השכחה
The verse · Devarim 24:19-20
כִּי תִקְצֹר קְצִירְךָ בְשָׂדֶךָ וְשָׁכַחְתָּ עֹמֶר בַּשָּׂדֶה לֹא תָשׁוּב לְקַחְתּוֹ לַגֵּר לַיָּתוֹם וְלָאַלְמָנָה יִהְיֶה לְמַעַן יְבָרֶכְךָ ה׳ אֱלֹהֶיךָ בְּכֹל מַעֲשֵׂה יָדֶיךָ כִּי תַחְבֹּט זֵיתְךָ לֹא תְפַאֵר אַחֲרֶיךָ לַגֵּר לַיָּתוֹם וְלָאַלְמָנָה יִהְיֶה
“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. When thou beatest thy olive tree, thou shalt not go over the boughs again: it shall be for the stranger, for the fatherless, and for the widow.”
The Rambam’s words
To leave a forgotten sheaf.
להניח עומר השכחה.
Why · Sefer HaChinuch #592
To leave what is forgotten to the poor
“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.”
Who & when
“And this commandment is practiced by males and females in the places that priestly tithes and tithes are practiced.”
Where in the Mishneh Torah
Hilchot Gifts to the Poor הלכות מתנות עניים