Airtel — Postpaid Refund Not Initiated By Airtel

I have an excess amount in my Airtel postpaid which eventually i have paid during[protected].. when i have asked Airtel to refund it.. Airtel is demanding for my bank statement of the period for the refund. Why would i need to give my personal bank statement to Airtel for the excess amount in my number. Under which policy they are forcing me for a bank statement.