“With a note of music, one strikes the fundamental, and, in addition to the root note, other notes are generated: these are called the harmonic series…As one fundamental note contains within it other notes in the octave, two fundamentals produce a remarkable array of harmonics, and the number of possible combinations between all the notes increases phenomenally. With a triad, affairs stand a good chance of getting severely out of hand…”

As quoted in Turning Numbers into Knowledge: Mastering the Art of Problem Solving (2001) by Jonathan G. Koomey, p. 96; since at least 2010 similar statements are also sometimes attributed to Mozart, and a similar remark, apparently one of Ben Jonson, is quoted in "Notes to Cynthia's Revels, in The Works of Ben Jonson: With Notes Critical and Explanatory, and a Biographical Memoir (1875), edited by William Gifford, Vol. 2, in notes to p. 223, on p. 551: Division, in music, is "the space between the notes of music, or the dividing of the tones."
Unsourced variants:
Music is the silence between the notes.
The music is not in the notes, but in the silence between.
The music is not in the notes, but in the silence between them.
Variant: Music is the space between the notes.

BBC Interview with Cecil Day Lewis (13 September 1957); transcripts published in "It Takes a Hero to Make a Poem" in the Claremont Quarterly (Spring 1958) http://www.frostfriends.org/FFL/Periodicals/Interview-lewis.html
1950s

Source: 1970s and later, Themes and Conclusions (1982), p. 52 : Dialogues.

Lecture IV: The Crystal Orders, section 35
The Ethics of the Dust (1875)

Awakening : Conversations with the Masters (2003), p. 24
Context: The genius of a composer is found in the notes of his music; but analyzing the notes will not reveal his genius. The poet's greatness is contained in his words; yet the study of his words will not disclose his inspiration. God reveals himself in creation; but scrutinize creation as minutely as you wish, you will not find God, any more than you will find the soul through careful examination of your body.

The Tragic Sense of Life (1913), III : The Hunger of Immortality

Nicomachus gives another reason for the name, viz. that a cube being of 3 equal dimensions, was the pattern &#940;&rho;&mu;&omicron;&nu;&#943;&alpha;: and having 12 edges, 8 corners, 6 faces, it gave its name to harmonic proportion, since:<center>12:6 :: 12-8:8-6</center>
Footnote, citing Vide Cantor, Vorles [Vorlesüngen über Geschichte der Mathematik ?] p 152. Nesselmann p. 214 n. Hankel. p. 105 sqq.
A Short History of Greek Mathematics (1884)

The Kate Bush Story (2014)
Context: You can hear one note of a Kate Bush song, or one note of her voice even, and know immediately what it is. And that is the biggest feat of any artist, especially when you consider, you know, all the roads that she's gone down.