Exodus 12:15, 39

[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.

And they baked unleavened cakes of the dough which they brought from Egypt; it was not leavened, since they were driven [quickly] from Egypt and could not delay, nor had they prepared any food for themselves.