Exodus 4:21-26

21 Yahweh said to Moses, "When you go back into Egypt, see that you do before Pharaoh all the wonders which I have put in your hand, but I will harden his heart and he will not let the people go.  22 You shall tell Pharaoh, 'Thus says Yahweh, Israel is my son, my firstborn,  23 and I have said to you, "Let my son go, that he may serve me;" and you have refused to let him go. Behold, I will kill your son, your firstborn.'"

24 It happened on the way at a lodging place, that Yahweh met him and wanted to kill him.  25 Then Zipporah took a flint, and cut off the foreskin of her son, and cast it at his feet; and she said, "Surely you are a bridegroom of blood to me."

26 So he let him alone. Then she said, "You are a bridegroom of blood," because of the circumcision.