Yes if not filed the GSTR 1 then do declare it in your GSTR 1 . I think you have filed your 3B without considering that bill then don't worry you can report it in your subsequent month 3B
Show that sale invoice under next month GSTR - 3B return, that is totally fine. Do not forget to declare in GSTR - 1 return of that period in which sale actually happened (if no GSTR - 1 return is filed)