Deuteronomy 16:3-4

You shall eat no leavened bread with it; seven days you shall eat unleavened bread with it, that is, the bread of affliction (for you came out of the land of Egypt in haste), that you may remember the day in which you came out of the land of Egypt all the days of your life. And no leaven shall be seen among you in all your territory for seven days, nor shall any of the meat which you sacrifice the first day at twilight remain overnight until morning.