In the Bible the Passover is the annual observance instituted in Exodus 12, on the night the Israelites leave Egypt, in which a lamb is killed and its blood placed on the doorposts.
Exodus 12:13 in the King James Version reads:
And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.
The verse appears in Exodus 12, in the instructions God gives Moses on the night of the tenth plague. Read it in context at Exodus 12:13, or the full chapter at Exodus 12. Encyclopedia entry: en.wikipedia.org/wiki/Passover.