Exodus 12:15, 39

For this feast you must eat bread made without yeast for seven days. On the first day, you are to remove all the yeast from your houses. No one should eat any yeast for the full seven days of the feast, or that person will be cut off from Israel.

The Israelites used the dough they had brought out of Egypt to bake loaves of bread without yeast. The dough had no yeast in it, because they had been rushed out of Egypt and had no time to get food ready for their trip.