“Physics is to mathematics what sex is to masturbation.”

Source: Standup Comic

Source: Ready Player One

In answer to the question, "Masturbation for you is as good as sex?"
The Playboy interview (2010)

Elements de la géométrie de l'infini (1727) as quoted by Amir R. Alexander, Geometrical Landscapes: The Voyages of Discovery and the Transformation of Mathematical Practice (2002) citing Michael S. Mahoney, "Infinitesimals and Transcendent Relations: The Mathematics of Motion in the Late Seventeenth Century" in Reappraisals of the Scientific Revolution, ed. David C. Lindberg, Robert S. Westman (1990)

Inaugural lecture for his professorship of mathematical physics at the University of Utrecht (1913), as quoted by Davies, Mansel. Peter Joseph Wilhelm Debye: 1884-1966. Biographical Memoirs of Fellows of The Royal Society, Vol. 16 (1970).

Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, Vol. 123, No. 792 http://doi.org/10.1098/rspa.1929.0094 (6 April 1929)
Context: The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. It therefore becomes desirable that approximate practical methods of applying quantum mechanics should be developed, which can lead to an explanation of the main features of complex atomic systems without too much computation.

The Evolution of the Physicist's Picture of Nature (1963)
Context: Just by studying mathematics we can hope to make a guess at the kind of mathematics that will come into the physics of the future. A good many people are working on the mathematical basis of quantum theory, trying to understand the theory better and to make it more powerful and more beautiful. If someone can hit on the right lines along which to make this development, it may lead to a future advance in which people will first discover the equations and then, after examining them, gradually learn how to apply them.

[Martinus Veltman, Facts and mysteries in elementary particle physics, World Scientific, 2003, 981238149X, 15, https://books.google.com/books?id=CNCHDIobj0IC&pg=PA15]