Exodus 12:17-21

You shall also keep the Feast of Unleavened Bread, for on this very day I brought your multitudes out of the land of Egypt; therefore you shall keep 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.

For seven days there shall be no dough with yeast found in your houses; for whoever eats anything with yeast, that person shall be cut off from the congregation of Israel, whether he is a stranger or a native of the land.

You shall not eat anything with yeast; in all your dwellings you shall eat unleavened bread.’ ”

Then Moses called for all the elders of Israel and said to them, “Go and take for yourselves lambs according to your families, and slaughter the Passover lamb.