The 122nd mitzvah is that we are commanded to leave over the sheaves which were forgotten (shik'cho) during the harvest process.
The source of this commandment is G‑d's statement,1 "[When you reap your harvest] and forget a sheaf in the field, you may not go back for it. It must be left for the stranger, the orphan, and the widow."
This phrase "It must be left for the stranger, the orphan, and widow" constitutes the positive commandment to leave over [these forgotten sheaves]. It is similar to the phrase, "leave them,"2 which conveys the positive commandments of leket and pe'ah, as explained above.3
The Biblical requirement applies only in Eretz Yisroel.4
The details of this mitzvah are also explained in tractate Pe'ah.5
