“If we have a correct theory but merely prate about it, pigeonhole it and do not put it into practice, then that theory, however good, is of no significance.”

Attributed in Nassim Nicholas Taleb, Antifragile - Things that Gain From Disorder (2012), p. 213.
The earliest known appearance of this quote in print is Walter J. Savitch, Pascal: An Introduction to the Art and Science of Programming (1984), where it is attributed as a "remark overheard at a computer science conference". It circulated as an anonymous saying for more than ten years before attributions to Jan L. A. van de Snepscheut and Yogi Berra began to appear (and later still to various others).
Disputed, Misattributed

The earliest known appearance in print of this quote is Benjamin Brewster in the October 1881 - June 1882 issue of "The Yale Literary Magazine." Brewster asks, "What does his lucid explanation amount to but this, that in theory there is no difference between theory and practice, while in practice there is?" See page 202. https://books.google.com/books?id=iJ9MAAAAMAAJ&printsec=frontcover&vq=%22no+difference%22#v=onepage&q&f=false It has also been attributed by Doug Rosenberg and Matt Stephens (2007) Use Case Driven Object Modeling with UMLTheory and Practice p. xxvii as well as Walter J. Savitch, Pascal: An Introduction to the Art and Science of Programming (1984), where it is attributed as a "remark overheard at a computer science conference". It circulated as an anonymous saying for more than ten years before attributions to van de Snepscheut and Yogi Berra began to appear (and later still to various others).
Misattributed

Preface (1961) p. vi; Partly cited by Stephen E. Robertson (2011) " On retrieval system theory http://www.iskouk.org/conf2011/papers/robertson.pdf".
On Retrieval System Theory (1961)

Source: How to Fall in Love

in his review of Joseph Beskiba's textbook, published in the Österreichische Blätter für Literatur und Kunst (September 7, 1844), as quoted by [Peter Schuster, Moving the stars: Christian Doppler, his life, his works and principle, and the world after, Living edition, 2005, 3901585052, 78]

Cultivation
One Minute Wisdom (1989)

Michel Henry, Marx I. une philosophie de la réalité, éd. Gallimard, coll. « Nrf », 1976, p. 353
Books on Economy and Politics, Marx. A Philosophy of Human Being (1976)
Original: (fr) Parce que la pratique est subjective, la théorie qui est toujours la théorie d’un objet, ne peut atteindre la réalité de cette pratique, ce qu’elle est en elle-même, sa subjectivité précisément, mais seulement se la représenter, de telle manière que cette représentation laisse hors d’elle l’être réel de la pratique, l’effectivité du faire. La théorie ne fait rien.

"The Face of Miranda", p. 496
Bully for Brontosaurus (1991)

Examples of self-translation (c. 2004), Quotes - Zitate - Citations - Citazioni