Dear Surendar,
If you read my answer once again, you'll understand that I've already answered this. Any leave is subject to prior sanction means that it has to be applied for in advance. How one has to apply depends on the type as well as the no. of days of leave intended to be availed of by the employee, the format of application, the intermediary channel of administrative hierarchy through which the application to be submitted, the alternative arrangement etc., are procedural formalities which the employer has to prescribe in the leave manual of the organization. Coming to the availment of casual leave, chances are there that enroute to office/place of work, an employee may fall sick suddenly and may be compelled to apply for C.L orally by phone and submit his written application later. Similarly, he may plan to attend a marriage that is to take place the next week and hence he may apply for 3 days C.L in advance for the sake of confirmation of prior sanction. Thus availment of C.L is purely a matter either of convenience or of emergency, procedural aspects of submission of application for and sanction are matters left for logical discretion only. Therefore, one can not search for any precise prescription in such matters in a Statute or its Rules.