Exodus 12:15, 39

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

They baked the dough which they had brought out of Egypt into cakes of unleavened bread. For it had not become leavened, since they were driven out of Egypt and could not delay, nor had they prepared any provisions for themselves.