How to use At command ???

Dear Sirs/Madams,

Can anybody explain me how to use AT USER-COMMNAD

in classical report...

I mean to say there must be a fix place where i need to write this code into the classical program.. and i dont know where should i write it..

for further information i am writing my bit code here :

REPORT YAGEINGCLASSICAL.

DATA : ITAB TYPE STANDARD TABLE OF STRING WITH HEADER LINE.

START-OF-SELECTION.

SET PF-STATUS 'AGEING'.

AT USER-COMMNAD.

BREAK-POINT.

CASE SY-UCOMM.

WHEN 'EXIT'.

LEAVE PROGRAM.

WHEN 'BACK'.

LEAVE TO SCREEN 0.

ENDCASE.

But it throws me an error..:

"AT USER-COMMAND" is allowed only within "LOOP...ENDLOOP".

howww??????