[...] All that I hear about you induces me to believe that in you I have found the singer I have sought in fear and trembling for my "Siegfried." As soon as you get to know this work (on which I am still engaged) you will easily be able to judge how critically important this question is to me. As far as I am concerned the whole should be ready for performance—if I am not interrupted by serious illness—by the summer of 1859. [...] My chief anxiety is as to whether I shall be fortunate enough to find the kind of performers I want. You are the first I have in view, which is the reason for my approaching you so soon. And yet you are causing me anxiety already, as I hear that you are one of those unusual people who sacrifice themselves to their zeal. Therefore take it kindly when I earnestly beseech you to bear in mind the extraordinary undertaking for which I solicit you and do not squander your powers. Take care of yourself and always remember that you have a great and noble artistic effort in front of you which, I promise you, will be worth present consideration.—[...]
Source: Letters of Richard Wagner, Vol. 1, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 326