Something along these lines
"sculpture is that which is done by taking away

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Note added at 18 mins (2017-06-03 13:15:54 GMT)
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Michelangelo took a different perspective. He considered the process of modeling in clay an “adding on” something to the marble that was not inherent in the stone itself. Michelangelo once wrote that a true and pure work of sculpture — by definition, one that is cut, not cast or modeled – should, as much as possible, retain much of the original form of the stone block and should avoid projections and separation of parts into something it is not.

In a letter from 1549, Michelangelo defined sculpture as the art of “taking away” not that of “adding on.” Michelangelo’s choice of marble block was key to his sculptural process. He believed that the form was already imprisoned in the stone and only needed to be discovered by the artist by taking away what was not needed to free the form.
- See more at: http://creativethinking.net/what-i-learned-about-creative-th...

If you look at this footnote there is a reference to the letter of 1549

https://books.google.it/books?id=JBYnVv2lKY4C&pg=PA223&lpg=P...

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Note added at 19 mins (2017-06-03 13:16:33 GMT)
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"sculpture is the art of taking away"

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Note added at 29 mins (2017-06-03 13:26:33 GMT)
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If you can find this book you may read the actual translation of the letter to Varchi

There is an English translation of Michelangelo
’
s 1549
letter to Benedetto Varchi on the hierarchy of sculpture and painting in
Complete Poems and Selected
Letters of Michelangelo
, trans. Creighton Gilbert and ed. Robert N. Linscott (New York, 1965), 283

The citation is als in this pdf

https://arthum.college.columbia.edu/sites/default/files/PDFs...
–
85; for a discussion of the letter, see David Summers,
Michelangelo and the Language of Art
(Prince-
ton, NJ, 1981), 269
–
73.