Deuteronomy 16:3-4

Thou shalt not eat with it leavened bread: seven days shalt thou eat without leaven, the bread of affliction, because thou camest out of Egypt in fear: that thou mayst remember the day of thy coming out of Egypt, all the days of thy life.

No leaven shall be seen in all thy coasts for seven days, neither shall any of the flesh of that which was sacrificed the first day in the evening remain until morning.