“Mechanical knowledge may… be distinguished into three kinds; purely scientific knowledge, purely practical knowledge, and that intermediate kind of knowledge which relates to the application of scientific principles to practical purposes, and which arises from understanding the harmony of theory and practice.”

On Practice (1937)

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)
Context: The objects of instruction in purely scientific mechanics and physics are, first, to produce in the student that improvement of the understanding which results from the cultivation of natural knowledge, and that elevation of mind which flows from the contemplation of the order of the universe; and secondly, if possible, to qualify him to become a scientific discoverer.<!--p. 176

Concluding Paragraph
On Practice (1937)
Original: (zh-CN) 通过实践而发现真理，又通过实践而证实真理和发展真理。从感性认识而能动地发展到理性认识，又从理性认识而能动地指导革命实践，改造主观世界和客观世界。实践、认识、再实践、再认识，这种形式，循环往复以至无穷，而实践和认识之每一循环的内容，都比较地进到了高一级的程度。这就是辩证唯物论的全部认识论，这就是辩证唯物论的知行统一观。

Source: Meeting the challenge (2009), p. xxiii.

Source: The Postmodern Condition: A Report on Knowledge (1977), p.7

"On the Harmony of Theory and Practice in Mechanics" (Jan. 3, 1856)
Context: Another evil, and one of the worst which arises from the separation of theoretical and practical knowledge, is the fact that a large number of persons, possessed of an inventive turn of mind and of considerable skill in the manual operations of practical mechanics, are destitute of that knowledge of scientific principles which is requisite to prevent their being misled by their own ingenuity. Such men too often spend their money, waste their lives, and it may be lose their reason in the vain pursuits of visionary inventions, of which a moderate amount of theoretical knowledge would be sufficient to demonstrate the fallacy; and for want of such knowledge, many a man who might have been a useful and happy member of society, becomes a being than whom it would be hard to find anything more miserable.
The number of those unhappy persons — to judge from the patent-lists, and from some of the mechanical journals — must be much greater than is generally believed.<!--p. 176

Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book I, Chapter I "The Education of the Architect" Sec. 1

As quoted by George H. W. Bush in remarks while presenting National Medals of Science and Technology http://bushlibrary.tamu.edu/research/papers/1990/90111300.html (13 November 1990). This might be a paraphrase of statements from his introduction to "Science The Endless Frontier" (1945), rather than a direct quote. (see below)

Max Weber, “Objectivity in Social Science and Social Policy” (1904)