Exodus 12:21-23

The Voice

Moses: Go and pick out lambs for each of your families, and then slaughter your family’s Passover lamb. ²² Take a handful of hyssop branches, dip them down into the bowl of blood you drained from the sacrifice, and mark the top of the doorway and the two doorposts with blood from the bowl. After you do this, no one should go out that door until the next morning.

²³ The Eternal will pass through the land during the night and bring death to the Egyptians. But when He sees the blood-markings across the tops of your doorways and down your two doorposts, He will pass over your houses and not allow His messenger of death to enter into your houses and strike you down.