EXO 12:18

18 In the first month of the year, on the 14th day of that month, the only bread you may eat is bread that has no yeast in it. You must keep doing that each day until the 21st day of that month. For those seven days you must not have any yeast in your house. [During that time], if anyone, either an Israeli or a foreigner, eats bread made with yeast, you must consider that person no longer to be an Israeli.