Shemot 12:12-13

For I will pass through Eretz Mitzrayim balailah hazeh, and will strike fatally kol bechor Eretz Mitzrayim, both adam and behemah; and against all the elohei Mitzrayim I will execute judgment; I am HASHEM. And the dahm shall be for you, an ot (sign) upon the batim (houses) where ye are; and when I see the dahm, oofasachti (then I will pass over, skip, spare) you [plural], and the negef (plague, blow, striking, i.e., death of firstborn) shall not be upon you to destroy you, when I strike fatally with a blow against Eretz Mitzrayim.