Exodus 12:1-7, 13

The LORD said to Moses and Aaron in the land of Egypt, “This month is to be the beginning of months for you; it is the first month of your year. Tell the whole community of Israel that on the tenth day of this month they must each select an animal of the flock according to their fathers’ families, one animal per family. If the household is too small for a whole animal, that person and the neighbor nearest his house are to select one based on the combined number of people; you should apportion the animal according to what each will eat. You must have an unblemished animal, a year-old male; you may take it from either the sheep or the goats. You are to keep it until the fourteenth day of this month; then the whole assembly of the community of Israel will slaughter the animals at twilight. They must take some of the blood and put it on the two doorposts and the lintel of the houses where they eat them.

The blood on the houses where you are staying will be a distinguishing mark for you; when I see the blood, I will pass over you. No plague will be among you to destroy you when I strike the land of Egypt.