This IRS requires that you have a receipt or statement showing that you paid for the medical expense. The explanation of benefits from the insurance company showing your payment responsibility does not prove that you paid it. All of these work: a copy of the cancelled check from bank website, receipt from medical provider showing payment, bank statement showing a debit card payment or credit card statement showing it was charged on your credit card (or the receipt from the merchant card machine), any other document showing your payment.
