Deuteronomy 16:3, 4, 8
Deuteronomy 16:4 NIV
Let no yeast be found in your possession in all your land for seven days. Do not let any of the meat you sacrifice on the evening of the first day remain until morning.
Deuteronomy 16:8 NIV
For six days eat unleavened bread and on the seventh day hold an assembly to the LORD your God and do no work.