Genesis 4:15; Genesis 4:24

15 And the Lord said to him: No, it shall not so be: but whosoever shall kill Cain, shall be punished sevenfold. And the Lord set a mark upon Cain, that whosoever found him should not kill him.
24 Sevenfold vengeance shall be taken for Cain: but for Lamech seventy times sevenfold.