Deuteronomy 16:3-4

Eat all of the meat of the Passover sacrifice that same night. But don't serve bread made with yeast at the Passover meal. Serve the same kind of thin bread that you ate when you were slaves suffering in Egypt and when you had to leave Egypt quickly. As long as you live, this thin bread will remind you of the day you left Egypt. For seven days following Passover, don't make any bread with yeast. In fact, there should be no yeast anywhere in Israel.