[...] I am sorry that you should have felt obliged to take the step of "dunning" me for Siegfried. The way The Valkyrie was "executed" (appropriate word!) in Munich this year, coupled with the discovery that my most illustrious patron was only waiting the arrival of the Siegfried score to subject it to a similar experiment, very seriously disinclined me to finish the latter work. As I am quietly working on very different schemes of my own for the production of the whole Nibelung cycle on my own lines, I feel more drawn at the moment to composing the last piece, The Twilight of the Gods, which is already fairly well advanced. Nevertheless, I have been working on the score of the last act of Siegfried as well, and three weeks ago the first half of it was dispatched to Klindworth. The rest will follow by the New Year. If you and he put your best foot forward then, both full score and pianoforte arrangement may quite well be out by Easter. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 644