According to the Midrash Pirke R. El. xxix., it was Shem who circumcised Abraham and Ishmael on the Day of Atonement; and the blood of the covenant then shed is ever before God on that day to serve as an atoning power. According to the same midrash, Pharaoh prevented the Hebrew slaves from performing the rite, but when the Passover time came and brought them deliverance, they underwent circumcision, and mingled the blood of the paschal lamb with that of the Abrahamic covenant, wherefore (Ezek. xvi. 6) God repeats the words: "In thy blood live!"