“Reading is not like listening to music, but rather like making music. The reader himself is the instrument.”
— Martin Walser
The quote in the original German is “Lesen ist nicht etwas wie Musikhören, sondern wie Musizieren. Das Instrument ist man selbst.” It comes from his book Des Lesers Selbstverständnis, p.12, and I found it quoted in translation in the essay by Manfred Oeming in Between Text and Text: International Symposium on Intertextuality in Ancient Near Eastern, Ancient Mediterranean, and Early Medieval Literatures, p.165.