Deuteronomy 16:3-4

3Eat it with bread made without yeast. For seven days the bread you eat must be made without yeast, as when you escaped from Egypt in such a hurry. Eat this bread—the bread of suffering—so that as long as you live you will remember the day you departed from Egypt.

4Let no yeast be found in any house throughout your land for those seven days. And when you sacrifice the Passover lamb on the evening of the first day, do not let any of the meat remain until the next morning.