Exodus 12:12-13

The blood shall be a sign for you, on the houses where you are.
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.

Exodus 12 12 13

Illustration from a watercolour by James Tissot (1836-1902) - Signs on the Door, c.1896-1902. Detail.