The 158th mitzvah is that we are commanded to eat matzah on the night of the 15th1 of Nissan. This mitzvah applies regardless of whether or not we have the Pesach sacrifice.2

The source of this commandment is G‑d's statement,3 "In the evening [of the 15th of Nissan] you must eat matzos."

Our Sages explicitly stated,4 " 'In the evening you must eat matzos' — the Torah establishes it as a requirement." It is explained in Pesachim that eating matzah on the first night of Pesach is a requirement, while afterwards it is optional.5

The details of this mitzvah are explained in tractate Pesachim.