“The genius of a composer is found in the notes of his music; but analyzing the notes will not reveal his genius.”

Source: Linden Hills

Source: The Romantic Generation (1995), Ch. 12 : Schumann: Triumph and Failure of the Romantic Ideal

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

Donald Trump, America’s first independent president (November 19, 2016)

1830s, The American Scholar http://www.emersoncentral.com/amscholar.htm (1837)

Source: Sex and Character (1903), p. 112.

Source: City of Thieves

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.