Exodus 12:13; Exodus 30:12

13 'The blood shall be a sign for you on the houses where you live; and when I see the blood I will pass over you, and no plague will befall you to destroy you when I strike the land of Egypt.

12 "When you take a census of the sons of Israel to number them, then each one of them shall give a ransom for himself to the LORD , when you number them, so that there will be no plague among them when you number them.