The 121st mitzvah is that we are commanded to leave over leket [stalks1 of grain which have fallen during the harvesting process].
The source of this commandment is G‑d's statement,2 "Do not pick up the stalks which fall during harvest. You must leave them for the poor and the stranger."3
This mitzvah is also in the category of lav shenitak l'aseh (a prohibition with a remedial positive commandment), as explained in tractate Makkos regarding pe'ah.4
The details of this mitzvah are explained in tractate Pe'ah.5
The Biblical prohibition applies only in Eretz Yisroel.6
