Exodus 13:15

and, when Pharao hardened himself against sending us away, the Lord slew every first-born in the land of Egypt, both the first-born of men and the first-born of cattle, therefore I sacrifice all firstlings—all the males to the Lord: and every male first-born of my children I must redeem.

And when Pharao hardened his heart so as not to send us away, he slew every first-born in the land of Egypt, both the first-born of man and the first-born of beast; therefore do I sacrifice every offspring that opens the womb, the males to the Lord, and every first-born of my sons I will redeem.