The 156th mitzvah is that we are commanded to remove chometz [leaven] from our possession on the 14th of Nissan. This is the mitzvah of "removing the leaven."
The source of this commandment is G‑d's statement (exalted be He),1 "On the day before [Pesach] you must remove the leaven from your homes."
Our Sages called this mitzvah, "biyur" ["clearing out"]; i.e., clearing out the chometz [from his possession].
The Jerusalem Talmud, tractate Sanhedrin2 says, "For possessing chometz one violates both a positive and a negative commandment. The positive commandment is biyur, as the verse says, 'you must remove the leaven from your homes'; the negative commandment is, 'no leaven may be found in your homes.' "3
The details of this mitzvah are explained in the beginning of tractate Pesachim.
