Deuteronomy 16:3-4

3Do not eat it with bread made with yeast, but for seven days eat unleavened bread, the bread of affliction, because you left Egypt in haste—so that all the days of your life you may remember the time of your departure from Egypt.

4Let no yeast be found in your possession in all your land for seven days. Do not let any of the meat you sacrifice on the evening of the first day remain until morning.