Full translation: "Much is permitted to the poet, the painter, and the fool! Gustav Klimt" (transl.). In 1913, the year Klimt penned this quotation, he completed his painting, "Die Jungfrau" which hangs in the National Gallery of Prague. The quotation is written on cream colored paper that has evenly aged. Additional images available on request.
In our search, we have not identified the source of the quote as other than Klimt himself.