Exodus 12:17-20

You shall also observe the Feast of Unleavened Bread, because on this very day I brought your hosts [grouped according to tribal armies] out of the land of Egypt; therefore you shall observe this day throughout your generations as an ordinance forever. In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, [and continue] until the twenty-first day of the month at evening. Seven days no leaven shall be found in your houses; whoever eats what is leavened shall be cut off and excluded from [the atonement made for] the congregation of Israel, whether a stranger or native-born. [1 Cor 5:6-8] You shall eat nothing leavened; in all your dwellings you shall eat unleavened bread.’ ”