Deuteronomy 16:3
Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread therewith, even the bread of affliction; for thou camest forth out of the land of Egypt in haste: that thou mayest remember the day when thou camest forth out of the land of Egypt all the days of thy life.
The command stands within the directions given for keeping the Passover from year to year. That Passover was to be commemorated by the Jewish nation. It held its place among their special religious observances.