Shemot (Exodus) 12:15
For a seven – day period you shall eat Mazzot, but on the previous day you shall eliminate leaven from your homes; for anyone who eats Chametz (leavened food) – that soul shall be cut off from Israel, from the first day to the seventh day.
CHAMETZ refers to any product made of wheat, rye, spelt, oats or barley that has been mixed with water and allowed to become leavened. LEAVENING refers to fermentation causing a dough to rise. Although Pessach Mazzot are also made from water and wheat, a fermentation is not taking place. Mazzot are baked within 18 minutes. It is this kind of speed avoiding a fermentation process.
Sefer HaChinuch (the Book of Mitzvot) lists this Mitzvah De’oraita as number 9.