Genesis 4:14-15

14Behold, thou hast driven me this day from the face of the ground, and from thy face shall I be hid; and I shall be a wanderer and fugitive on the earth; and it will come to pass, [that] every one who finds me will slay me.

15And Jehovah said to him, Therefore, whoever slayeth Cain, it shall be revenged sevenfold. And Jehovah set a mark on Cain, lest any finding him should smite him.