Exodus 4:22-26

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.

And it came to pass by the way in the inn, that the LORD met him, and sought to kill him.

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.