Exodus 12:15, 39

Seven days you shall eat unleavened bread. On the first day you shall remove leaven from your houses. For whoever eats leavened bread from the first day until the seventh day, that person shall be cut off from Israel.

And they baked unleavened cakes of the dough which they had brought out of Egypt; for it was not leavened, because they were driven out of Egypt and could not wait, nor had they prepared provisions for themselves.