“Simplicity is the final achievement. After one has played a vast quantity of notes and more notes, it is simplicity that emerges as the crowning reward of art.”

Source: The Hour of the Star

Source: My Life and Music (1961), p. 122

This is Your Brain on Music (2006)
Context: When a musical piece is too simple we tend not to like it, finding it trivial. When it is too complex, we tend not to like it, finding it unpredictable—we don't perceive it to be grounded in anything familiar. Music, or any art form... has to strike the right balance between simplicity and complexity...

Source: Life's Solution (2003), p. 315.

A Voice from the Attic (1960)

Deepa Ganesh, in "Gangubai's search for perfection"

A Conversation with Ward Cunningham (2003), The Simplest Thing that Could Possibly Work

Not found in Beethoven's known works. It may be a summary of the following description of Beethoven from his piano pupil Ferdinand Ries: "When I left out something in a passage, a note or a skip, which in many cases he wished to have specially emphasized, or struck a wrong key, he seldom said anything; yet when I was at fault with regard to the expression, the crescendo or matters of that kind, or in the character of the piece, he would grow angry. Mistakes of the other kind, he said were due to chance; but these last resulted from want of knowledge, feeling or attention. He himself often made mistakes of the first kind, even playing in public."
Disputed
Source: "When Beethoven gave me a lesson" https://books.google.com/books?id=j8RIq67v51cC&pg=PA294&dq=%22when+beethoven+gave+me+a+lesson%22&hl=en&sa=X&ved=0CBwQ6AEwAGoVChMI7Yyz0PiNyQIViDuICh1YIAzR#v=onepage&q=%22when%20beethoven%20gave%20me%20a%20lesson%22&f=false