[...] As you know, Hans wanted to perform the Overture to Tristan last winter, and asked me to make a close for it. At that time I had no idea for it, and it seemed so impossible that I flatly declined to do it. Since then I have written the third act and found a full close to the whole. The idea of intimating this close in advance came to me as a dawning presentiment of deliverance as I drew up a programme for my Parisian concert, the prospect of hearing the Tristan Overture played there being a special incentive to me. Well, it has turned out excellently and I am to-day sending you this mysterious, reassuring close, as the very best thing I can give you for your birthday. I have written the piece out for you very much as I play it to myself on the pianoforte. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 451