A proof of payment is a debit card/credit card slip clearly indicating that the payment has gone through and the transaction was approved. This may be the small slip that comes out of a debit/credit machine or it may be printed on the same paper as your receipt, as is the case at many large retailers.
If you have lost your proof of payment, go into your online banking and print off your statement or take a screenshot. Highlight the purchase and blackout any personal/misc. information.