You must declare all visa denials, but there is no requirement to send in documentation relating to it.
On the visitor application form VA1FA you are asked the date, the country AND the reason for the refusal.My family member had a US visa refused once, even though has had two US Visas granted since then. We've always used the words: "Refused a USA visa on DD MM YYYY." and have had no adverse issues with that refusal, having got UK and other visas. We've never needed to mention the REASON for the refusal nor supply any documentation.