How can I return to a selection screen?

Hello,

I have an ABAP program, which has, as the first screen, parameters. When I hit 'enter', the second screens appears.
However, when I click 'Back' it doesn't return to the selection screen.

Here's what I have:

SELECTION-SCREEN BEGIN OF BLOCK b1
                  WITH FRAME TITLE TEXT-001.
         PARAMETERS aux TYPE string.
SELECTION-SCREEN END OF BLOCK b1.

AT SELECTION-SCREEN.
   CALL SCREEN 101.

MODULE STATUS_0101 OUTPUT.
   SET PF-STATUS 'STATUS'.
   SET TITLEBAR 'TITLE'.

   "more code...

ENDMODULE.

MODULE USER_COMMAND_0101 INPUT.
   IF sy-ucomm = 'BACK'.
       FREE: oref_custom,
             oref_alv.
       LEAVE SCREEN.
   ENDIF.

   IF sy-ucomm = 'EXIT' OR sy-ucomm = 'CANCEL'.
     FREE: oref_custom,
                oref_alv.
     LEAVE PROGRAM.
   ENDIF.
ENDMODULE.

I don't know how to call the first screen again, since I didn't actually created it.

Thank you very much in advance.