“If the question [about if there were only one composer in the world I could play] is purely ‘original’ works, no transcriptions, then Beethoven.”

Heifetz official web site http://www.jaschaheifetz.com/about/quotes.html

Source: The Romantic Generation (1995), Ch. 10 : Mendelssohn and the Invention of Religious Kitsch

As quoted on Silvert, Conrad, “Joe Zawinul: Wayfaring Genius–Part II,” Down Beat, Jun. 15, 1978, pp. 20-22

19th June 1949, in Willi Schuh, Strauss the early years, 1982, Cambridge University Press, page xiii.
Other sources

47 : The Question and its Answer, p. 78.
The Everything and the Nothing (1963)

Part II. The Classical Style. 1. The Coherence of the Musical Language
Classical Style: Haydn, Mozart, Beethoven (Expanded edition, 1997)

Gramophone
Context: I think that if I were required to spend the rest of my life on a desert island, and to listen to or play the music of any one composer during all that time, that composer would almost certainly be Bach. I really can't think of any other music which is so all-encompassing, which moves me so deeply and so consistently, and which, to use a rather imprecise word, is valuable beyond all of its skill and brilliance for something more meaningful than that -- its humanity.

"Firestorm Grows Over Islamic Center Near Ground Zero" http://transcripts.cnn.com/TRANSCRIPTS/1008/17/acd.01.html, Anderson Cooper 360°, CNN (17 August 2010)