Deuteronomy 16:3-4

3You shall not eat leavened bread with it; instead, for seven days you shall eat the Passover with unleavened bread, the bread of affliction (for you left the land of Egypt in haste); [do this] so that all the days of your life you may remember [thoughtfully] the day when you came out of the land of Egypt.

4For seven days no leaven shall be seen with you in all your territory, and none of the meat which you sacrificed the evening of the first day shall remain overnight until morning.