From Alma Mahler's Gustav Mahler: Memories and Letters, page 182.
Eighth Symphony, 12th September 1910
We took up our quarters with my mother, as usual. One evening we asked Zemlinsky and Schoenberg in. Schoenberg took me aside. “I promise you,” he said, “never to argue with Mahler again. From today on he can shout at me as hard as he likes. I shall never take offence.” I was more alarmed than pleased. “My mind is made up,” he went on. “And it is because I love him.”
I remember a discussion Mahler once had with Schoenberg about the possibility of creating a melody from one note played successively on different instruments. Mahler strenuously denied that it could be done.