Genesis 4:13-15

And Cain said unto Jehovah, My punishment is greater than I can bear. Behold, thou hast driven me out this day from the face of the ground; and from thy face shall I be hid; and I shall be a fugitive and a wanderer in the earth; and it will come to pass, that whosoever findeth me will slay me. And Jehovah said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And Jehovah appointed a sign for Cain, lest any finding him should smite him.