Exodus 12:17-20

You shall also observe the Feast of Unleavened Bread, for on this very day I brought your hosts out of the land of Egypt; therefore you shall observe this day throughout your generations as a permanent ordinance. In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. Seven days there shall be no leaven found in your houses; for whoever eats what is leavened, that person shall be cut off from the congregation of Israel, whether he is an alien or a native of the land. You shall not eat anything leavened; in all your dwellings you shall eat unleavened bread.’ ”