Proof of purchase,

Definition of Proof of purchase:

1. Confirmation in the form of a receipt or other document that one has paid for a particular item.

2. Bottle cap, coupon, label, sale receipt, or certain part of a package, required by a promoter to qualify a customer for participating in a contest, receiving a discount or prize, or availing a special offer.

How to use Proof of purchase in a sentence?

1. We had to have proof of purchase in order to qualify for service under the warranty.