How do I instruct my custom GPT to refer to a document or link when giving its answer?

I have built a custom GPT in which I have uploaded 20 documents (the maximum). When a user interacts with the custom GPT, how would I configure it to refer to one of its documents (or an HTTP link if possible) when giving its answer. I tried adding these instructions

Please provide a concise summary or answer based on the information extracted from the referenced document or link. Ensure to include a brief mention or citation of the source material for further reference.

but the GPT does not include any mention of where it got its information in its response.

I tried adding the following policy rules it worked

“Always refer to documents.pdf for any queries related to the product summary”