Exodus 12:18-20

18In 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.

19Seven 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.

20You shall eat nothing leavened; in all your dwellings you shall eat unleavened bread.’ ”