[...] Even before you wrote to me I had assigned you a function in my projected enterprise of very great importance for the future. As a matter of fact, I wished to ask you to follow all my rehearsals, just as you did for the Ninth Symphony, and to note and write down every remark of mine, however intimate, bearing on the interpretation and performance of our work, so that a fixed tradition might be established.
I hope this may be considered settled! But if you are willing to help me with the purely mechanical musical coaching of the individual singers, you will be very welcome to me. I cannot make any actual engagement in this respect, and all you may have read about it in newspaper paragraphs is vain boasting, which will do no good to the boasters. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 660