Deuteronomy 16:3-4

You shall eat no leavened bread with it. You shall eat unleavened bread with it seven days, even the bread of affliction; for you came out of the land of Egypt in haste; that you may remember the day when you came out of the land of Egypt all the days of your life.

No yeast shall be seen with you in all your borders seven days; neither shall any of the flesh, which you sacrifice the first day at evening, remain all night until the morning.