Parzival.
Wo find' ich dich, du heil'ger Gral, dich sucht voll Sehnsucht mein Herz.
Dear errant child!
See, I was just about to write this down, when I found thy lovely, noble verses.
Source: Richard Wagner to Mathilde Wesendonck, trans. William Ashton Ellis (Charles Scribner's Sons, 1905), No. 52