Negative commandment · Lo Ta’aseh #210 of 613
Not to harvest one's entire field
שלא לקצור כל השדה
The verse · Vayikra 23:22
וּבְקֻצְרְכֶם אֶת־קְצִיר אַרְצְכֶם לֹא־תְכַלֶּה פְּאַת שָׂדְךָ בְּקֻצְרֶךָ וְלֶקֶט קְצִירְךָ לֹא תְלַקֵּט לֶעָנִי וְלַגֵּר תַּעֲזֹב אֹתָם אֲנִי ה׳ אֱלֹהֵיכֶם
“And when you reap the harvest of your land, thou shalt not altogether remove the corners of thy field when thou reapest, nor shalt thou gather any gleaning of thy harvest: thou shalt leave them to the poor, and to the stranger: I am the Lord your God.”
The Rambam’s words
Not to harvest one's entire field, as [Leviticus 23:22] states: "Do not completely harvest the ends of your fields."
שלא לקצור כל השדה, שנאמר "לא תכלה פאת שדך בקוצרך" (ויקרא כג,כב).
Why · Sefer HaChinuch #217
To not finish the corner in the field
“It is from the roots of the commandment that G-d, blessed be He, wanted that His people that He chose be crowned with every good and dear trait and that they would have a blessed soul and a generous spirit.”
Where in the Mishneh Torah
Hilchot Gifts to the Poor הלכות מתנות עניים