Exodus 12:22-23

12:22 Take a branch of hyssop, dip it in the blood that is in the basin, and apply to the top of the doorframe and the two side posts some of the blood that is in the basin. Not one of you is to go out the door of his house until morning. 12:23 For the Lord will pass through to strike Egypt, and when he sees the blood on the top of the doorframe and the two side posts, then the Lord will pass over the door, and he will not permit the destroyer to enter your houses to strike you.