Bo: Chapter 9
Leaven and leavened dough

In this discussion, we read that leaven, leavened fermentation and leavened bread are all the same thing. In response to Rabbi Yehuda's query about why, if it is so important, Yisrael is only restricted from eating leaven seven days a year, Rabbi Shimon replies that this is an annual celebration of the days that they went out of bondage.