Exodus 12:18-20

In the first month 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 no yeast is to be found in your houses. And anyone, whether foreigner or native-born, who eats anything with yeast in it must be cut off from the community of Israel. Eat nothing made with yeast. Wherever you live, you must eat unleavened bread.”