Exodus 12:14-15

‘Now this day will be a memorial to you, and you shall keep it as a feast to the LORD; throughout your generations you are to celebrate it as an ordinance forever. [In the celebration of the Passover in future years,] seven days you shall eat unleavened bread, but on the first day you shall remove the leaven from your houses [because it represents the spread of sin]; for whoever eats leavened bread on the first day through the seventh day, that person shall be cut off and excluded from [the atonement made for] Israel.