Deuteronomy 16:3-4

3Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread therewith, even the bread of affliction; for thou camest forth out of the land of Egypt in haste: that thou mayest remember the day when thou camest forth out of the land of Egypt all the days of thy life.

4And there shall be no leaven seen with thee in all thy borders seven days; neither shall any of the flesh, which thou sacrificest the first day at even, remain all night until the morning.