"You shall not offer the blood of my sacrifice with leavened bread"Exodus 23:18.

It is forbidden to have chametz (leavened bread) in one's possession from the onset of the time when the Paschal Offering is brought—noon of the fourteenth of Nissan [the eve of Passover].

This prohibition applies to all those who are participating in the eating of the sacrifice, the priest who slaughters it, the priest who sprinkles its blood on the altar, and the priest who offers its fats on the altar.