Exodus 12:13-15

13The blood on the houses where you are staying will be a sign; when I see the blood, I will pass over you. No plague will fall on you to destroy you when I strike the land of Egypt.

14And this day will be a memorial for you, and you are to celebrate it as a feast to the LORD, as a permanent statute for the generations to come. 15For seven days you must eat unleavened bread. On the first day you are to remove the leaven from your houses. Whoever eats anything leavened from the first day through the seventh must be cut off from Israel.