From Alma Mahler's Gustav Mahler: Memories and Letters, page 279: Letters 1906
Arnold Schoenberg to Gustav Mahler
14 June [? April, May] 1906
My dear Director,
I am not quite sure yet whether the engagement at the Opera refers to Zemlinsky, or myself. However, that is a matter of indifference and was not really what I wanted to say at all. What I did want to say was how deeply impressed I was by the magnificence of the work I heard today. (probably Mahler's Sixth Symphony) I cannot forbear telling you that such music could come from only one man in the whole world and that man is: Mahler. I have always held you very, very dear - only you have perhaps not known it - but today I know why.
I kiss your hands a thousand times.