The basic rule is this:
Quote Quoting California Civil Code, Sec. 1499.
A debtor has a right to require from his creditor a written receipt for any property delivered in performance of his obligation.
Are you stating that you were asking for receipts at the time of your past payments but that your landlord has declined to give you receipts? (If so, without excusing your landlord, consider using a payment method that creates a record, such as checks or money orders.)

Or is it that your landlord is saying, "If you want a receipt, you need to ask for it when you make your payment," and they don't want to issue receipts for past rent payments for which no contemporaneous request was made? Have they (or you) suggested an alternative, such as a copy (or printout, if it's computerized) of the payment ledger for your unit?