Amazon India - Cashback not recived

I brought a mobile in July ** **** , it is having a offer of **% cashback only if I buy using hdfc credit card through online or we can order cash on delivery and we can pay using hdfc credit card, that should credit with in October ** but it's now end of the November month , still I didn't revived my cash back , I called them * times they told we created complaint it will credit with in ** hours ,after * week again for I called they told they want bank statement, but I brought using my room mate credit card ,he told I can give my bank statement it's confidential, but amazon is saying what is the proof that u paid using credit and they it's credited I asked for reference number but they told it's not updated no reference number and alll