Exodus 12:13-15

13And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and there shall no plague be upon you to destroy you, when I smite the land of Egypt. 14And this day shall be unto you for a memorial, and ye shall keep it a feast to Jehovah: throughout your generations ye shall keep it a feast by an ordinance for ever.

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