Leviticus 23:6
"And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD: seven days ye must eat unleavened bread."
Cross References (ranked by relevance)
Exodus 34:18
The feast of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, in the time of the month Abib: for in the month Abib thou camest out from Egypt.
Deuteronomy 16:8
Six days thou shalt eat unleavened bread: and on the seventh day shall be a solemn assembly to the LORD thy God: thou shalt do no work therein.