Exodus 4:24

And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn:

And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.

Then Zipporah took a sharp stone, and cut off the foreskin of her son, and cast it at his feet, and said, Surely a bloody husband art thou to me.

So he let him go: then she said, A bloody husband thou art, because of the circumcision.