On authority of Anas (may Allah be pleased with him) who said I heard Rasulullah (saw) say Allah the Almighty said:
O SON OF ADAM, SO LONG AS YOU CALL UPON ME AND ASK OF ME,I SHALL FORGIVE YOU FOR WHAT YOU HAVE DONE, AND I SHALL NOT MIND. O SON OF ADAM, WERE YOUR SINS TO REACH THE CLOUDS OF THE SKY AND WERE YOU THEN TO ASK FORGIVENESS OF ME,I SHALL FORGIVE YOU. O SON OF ADAM,WERE YOU TO COME TO ME WITH SINS AS GREAT AS THE EARTH AND WERE YOU THEN TO FACE ME, ASCRIBING NO PARTNER TO ME, I WOULD BRING YOU FORGIVENESS NEARLY AS GREAT AT IT.
Related by At Thirmidhi also by Ahmad ibn Hanbal, Its chain of authorities is sound.