How to procure legal heir certificate for FD purposes ?

After death of my father i found 3 fixed deposits in his locker, all of three fixed deposits nominee are my mother, she is died before my father, and i am the joint account holder of my father savings account. Now i am only heir in my family after death of my father, with this background when I ask in bank about those fixed deposits, they say you have required a legal heir certificate those prove you are the only heir after your father. So please guide me how to i get the certificate and what the procedure for this.