Exodus 12:8, 18, 34, 39

That same night they are to eat the meat roasted over the fire, along with bitter herbs, and bread made without yeast.

In the first month you are to eat bread made without yeast, from the evening of the fourteenth day until the evening of the twenty-first day.

So the people took their dough before the yeast was added, and carried it on their shoulders in kneading troughs wrapped in clothing.

With the dough the Israelites had brought from Egypt, they baked loaves of unleavened bread. The dough was without yeast because they had been driven out of Egypt and did not have time to prepare food for themselves.