Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 124

להניח פרט הכרם לפי שבכל אלו נאמר "לֶעָנִי וְלַגֵּר תַּעֲזֹב אֹתָם" (ויקרא יט, י) (ויקרא כג, כב) וזו היא מצות עשה שלהם.

To leave the individual fallen grapes. With regard to all these [five mitzvot], [Leviticus 19:10] states: "Leave them for the poor and the stranger." This [verse states] the positive commandment for all these. the Rambam notes that the command to leave a forgotten sheaf in the field is derived, not from this verse, but from Deuteronomy 24:19, which states: "When you reap your harvest... and forget a sheaf in the field..., it shall be for the stranger..."