"You shall eat no leavened bread upon it"—Deuteronomy 16:3.
We are forbidden to eat chametz (leavened foods) after midday of the fourteenth of Nissan. As the Torah says, "You shall eat no leavened bread upon it"—"it" referring to the Paschal Offering, which was offered after midday on the eve of Passover.
(The Sages extended this prohibition to include a time period before midday, to prevent anyone from inadvertently consuming chametz after midday, when it is biblically forbidden.)
