DEU 16:3

3 [When you eat the Passover meal], the bread that you eat must not have yeast in it. You must eat this kind of bread, which will be called the bread of suffering/misery, for seven days. This is to help you to remember all during the time that you are alive that when your ancestors left Egypt, [where they were suffering because they were slaves], they left very quickly. [They did not put in yeast and wait for the dough to swell up].