A voucher is an invoice, a receipt or other statement related to your business activity. The voucher shows what has been purchased, when, by what amount and the amount of VAT.
When buying a product or a service, you receive a bill (an invoice) from the supplier. This invoice, the original invoice that is, must be used for a voucher for the bookkeeping. This applies whether you buy cash or on credit - i.e. whether the payment is made on purchase or later.