Exodus 12:13-15

13And the blood shall be for you as a sign on the houses in which ye are; and when I see the blood, I will pass over you; and the plague shall not be among you for destruction, when I smite the land of Egypt.

14And this day shall be unto you for a memorial; and ye shall celebrate it [as] a feast to Jehovah; throughout your generations [as] an ordinance for ever shall ye celebrate it.

15Seven days shall ye eat unleavened bread: on the very first day ye shall put away leaven out of your houses; for whoever eateth leavened bread from the first day until the seventh day — that soul shall be cut off from Israel.