Four Torah commandments deal with the prohibition against ĥametz on Pesaĥ: three negative and one positive.
The first prohibition is to refrain from eating ĥametz, as it is written, “And ĥametz shall not be eaten” (Shemot 13:3). Our Sages taught that the prohibition against eating ĥametz on Pesaĥ includes not deriving any kind of benefit from the ĥametz. The Torah also says, “You shall not eat anything leavened; in all of your settlements you shall eat matzot” (Shemot 12:20). Our Sages concluded from this verse that not only something that had fermented on its own is prohibited, but even food that had been leavened by some external agent may not be eaten on Pesaĥ. It must be noted that the Torah was particularly stringent concerning the prohibition against eating ĥametz. Almost all of the Torah’s food prohibitions are punishable by lashes, while eating ĥametz on Pesaĥ is punishable by karet (extirpation), as it is written, “whoever eats ĥametz from the first day until the seventh day, that soul shall be cut off from Israel” (Shemot 12:15).
The second prohibition is that no ĥametz may be found in our possession, as it is written, “Seven days there shall be no se’or found in your homes” (Shemot 12:19). Se’or is the leavening agent that one uses to make dough ferment. This verse means not only that se’or is forbidden, but also that no ĥametz may be found in our possession on Pesaĥ. This prohibition is often called bal yimatzei.
The third prohibition is that no ĥametz may be seen in our possession, as it is written: “Matzot shall be eaten seven days; and no ĥametz of yours shall be seen, and no se’or of yours shall be seen within all your borders” (Shemot 13:7). One violates the second prohibition (bal yimatzei) and this third prohibition (called bal yera’eh) only if one has in his possession on Pesaĥ at least one olive’s bulk (kezayit) of ĥametz. If the volume of the ĥametz that remained in one’s possession was less than a kezayit, he does not violate bal yera’eh and bal yimatzei on account of that ĥametz.
The fourth mitzva – a positive commandment – is to get rid of ĥametz and se’or in advance of Pesaĥ, as it is written, “Seven days you shall eat matzot; however, on the first day you shall remove the se’or from your houses” (Shemot 12:15).