Exodus 12:21-23

Then Moses called for all the elders of Israel, and said unto them, Draw out, and take you lambs according to your families, and kill the passover. And ye shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two side-posts with the blood that is in the basin; and none of you shall go out of the door of his house until the morning. For Jehovah will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side-posts, Jehovah will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.