Deuteronomy 16:3-4

3Never eat leavened bread with the meat from this sacrifice. Instead, for seven days you must eat unleavened bread at this festival. (It is the bread of misery because you left Egypt in a hurry.) Eat this bread so that, as long as you live, you will remember the day you left Egypt.

4There should be no yeast anywhere in your land for seven days. Never leave until morning any of the meat you slaughter on the evening of the first day.