Deuteronomy 16:2-3

2Sacrifice as the Passover to the LORD your God an animal from your flock or herd at the place the LORD will choose as a dwelling for his Name. 3Do not eat it with bread made with yeast, but for seven days eat unleavened bread, the bread of affliction, because you left Egypt in haste—so that all the days of your life you may remember the time of your departure from Egypt.