Exodus 12:6, 18-20

Take care of them until the fourteenth day of the month, when all the members of the community of Israel must slaughter them at twilight.

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.”