Take the Qur’an
Abu Al-’Âliyah reports that a man once asked ‘Ubay b. Ka’b – Allah be pleased with him – for advice. He said:
Take the Book of Allah as your leader (imâm) and be pleased with it as a judge and ruler. It is what your Messenger left amongst you. It will be an intercessor for you. It is to be obeyed. It is a witness never doubted. In it is a mention of you and those before you, and judgment for whatever happens amongst you. And in it is news about you and whatever will come after you.
Al-Dhahabî in Siyar A’lâm Al-Nubalâ’, in the biography of ‘Ubay b. Ka’b