… leavened products may not be seen in your possession and leavening agents may not be seen in your possession, wherever you may live… (Exodus 13:7)
This mitzvah actually prohibits two things, chometz (leavened items, like bread) and seor (leavening agents, such as yeast). The prohibition is “l’cha” – “to you,” i.e., in your possession. If the plumber comes on Chol HaMoed Pesach and he has a bologna sandwich, you’ll want to be sure that he doesn’t leave crumbs all over the place, but its mere presence in your house is not in and of itself problematic because it’s not “l’cha” – it doesn’t belong to you, it belongs to him. (This is why sold chometz is not problematic. Legally, it does not belong to the home owner in whose house it resides. Please note that the sale of chometz is not a loophole or a legal fiction; it must be a legally valid transaction under Jewish law, carried out conforming to all the pertinent halachos.)
In previous mitzvos pertaining to chometz on Pesach, we have addressed the various metaphors of eschewing leaven, such as comparisons of chometz to one’s ego or to idolatry.
This mitzvah interacts with mitzvah #11, the prohibition against finding chometz in one’s house, with each shedding some light on the other. Collectively, they are referred to as “bal yera’eh u’bal yimatzei,” “not seeing and not finding.” The Talmud discusses these in Pesachim on page 5b. If one does find chometz over Pesach it must be destroyed immediately on Chol HaMoed. If one finds it on Yom Tov, it must be covered until Yom Tov ends and then destroyed (OC 446:1).
This mitzvah applies to both men and women, in all times and places. In the Shulchan Aruch, it can be found in Orach Chaim 431, 440 and 446. It is #200 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #2 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.