That is a quotation from Lewis Carroll's 1871 book "Through the
Looking-Glass, and what Alice found there", the sequel to "Alice's
Adventures in Wonderland". The actual quotation, which can be found in
Chapter 1, differs only slightly from your version. It goes:
`The horror of that moment,' the King went on, `I shall never, never forget!'
`You will, though,' the Queen said, `if you don't make a memorandum of it.'
Motifs from "Through the Looking-Glass", combined with elements from
"Alice's Adventures in Wonderland", were indeed used for the Disney
cartoon "Alice in Wonderland" and for various picturizations of the
You can read the quote in context at Literature.org, where the full
novel is available online:
Hope this answers your question!
Wikipedia: Through the Looking-Glass
Literature.org: Through the Looking-Glass, Chapter 1
Search terms used:
"unless you make a memorandum of it"
"through the looking glass" memorandum