Deuteronomy 16:3-4

3Do not eat leavened bread with it. For seven days you are to eat unleavened bread with it, the bread of hardship — because you left the land of Egypt in a hurry  — so that you may remember for the rest of your life the day you left the land of Egypt.

4No yeast is to be found anywhere in your territory for seven days, and none of the meat you sacrifice in the evening of the first day is to remain until morning.