Exodus 12:17-20

You must celebrate the Feast of Unleavened Bread, because on this very day I brought your divisions of people out of Egypt. So all of your descendants must celebrate this day. This is a law that will last from now on. In the first month of the year you are to eat bread made without yeast, from the evening of the fourteenth day until the evening of the twenty-first day. For seven days there must not be any yeast in your houses. Anybody who eats yeast during this time, either an Israelite or non-Israelite, must be cut off from the community of Israel. During this feast you must not eat anything made with yeast. You must eat only bread made without yeast wherever you live.”