Exodus 12:21-22

21Then Moses called the leaders of the Israelites to come to him. He said to them, ‘Go immediately and choose a lamb for each of your families. Then kill the animals for the Passover feast.

22Put the blood into a dish. Then take a branch of hyssop. Make it wet with the blood that is in the dish. Then put some of the blood on the wood that is round your door. Nobody must go out of the door of his house until the morning.