Exodus 4:22-26

22Then say to Pharaoh, ‘This is what the LORD says: Israel is my firstborn son,

23and I told you, “Let my son go, so he may worship me.” But you refused to let him go; so I will kill your firstborn son.’ ” 24At a lodging place on the way, the LORD met Moses Hebrew him and was about to kill him. 25But Zipporah took a flint knife, cut off her son’s foreskin and touched Moses’ feet with it. The meaning of the Hebrew for this clause is uncertain. “Surely you are a bridegroom of blood to me,” she said.

26So the LORD let him alone. (At that time she said “bridegroom of blood,” referring to circumcision.)