Genesis 4:14-15

14Surely You have driven me out this day from the face of the ground; I shall be hidden from Your face; I shall be a fugitive and a vagabond on the earth, and it will happen that anyone who finds me will kill me.”

15And the Lord said to him, “Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold.” And the Lord set a mark on Cain, lest anyone finding him should kill him.