They are to take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs.

Exodus 12:22
Take a cluster of hyssop, dip it into the blood in the basin, and brush the blood on the top and sides of the doorframe. None of you shall go out the door of his house until morning.

Exodus 12:23
When the LORD passes through to strike down the Egyptians, He will see the blood on the top and sides of the doorframe and will pass over that doorway; so He will not allow the destroyer to enter your houses and strike you down.

Deuteronomy 11:20
Write them on the doorposts of your houses and on your gates,

1 Kings 6:31
For the entrance to the inner sanctuary, Solomon constructed doors of olive wood with five-sided doorposts.

Ezekiel 9:4
"Go throughout the city of Jerusalem," said the LORD, "and put a mark on the foreheads of the men sighing and groaning over all the abominations committed there."

And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it.