The portion of the credit card purchases that you receive back in cash does not constitute gross income (i.e., not taxable).
However, if the purchase was a business expense, you cannot deduct the portion you receive in cash back from your business income (e.g., You purchase $500 in office supplies and receive $25 cash back. You can deduct $475 ($500 less the $25 "discount").
However, when used for business, what you basically have is a discount that reduces your cost of goods (your inventory cost). You should probably reduce your inventory cost by the amount of the rebate check (which will increase your net profit.) It is probably easier to report the check as income, which has the same result in the end.