Section 13(2)(a) in The Post Office Savings Bank General Rules, 1981
(a)If a nomination made under Rule -12 is in force at the time of death of the depositor of a single account or the surviving depositor in the case of joint account, the nominee or nominees surviving such depositor may make an application in the manner prescribed to the relevant Head Savings Bank for payment of the amount due on the account and such application shall be accompanied by proof of death of the depositor or depositors, as the case may be, and where any other nominee has predeceased the depositor, by proof of death of such minor.