Exodus 12:15, 39

Seven days you shall eat unleavened bread. On the first day you shall remove leaven out of your houses, for if anyone eats what is leavened, from the first day until the seventh day, that person shall be cut off from Israel.

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