“Somebody asked me the question not too long ago: 'Dave, do you think the music business has turned corrupt'. I said: 'Absolutely not - it has always been corrupt.”

The Guardian interview (2002)

Source: The Ordeal of Change (1963), Ch. 2: "The Awakening of Asia" This passage uses phrases from his earlier work The Passionate State Of Mind, and Other Aphorisms (1955)
Context: It has often been said that power corrupts. But it is perhaps equally important to realize that weakness, too, corrupts. Power corrupts the few, while weakness corrupts the many. Hatred, malice, rudeness, intolerance, and suspicion are the faults of weakness. The resentment of the weak does not spring from any injustice done to them but from the sense of inadequacy and impotence. We cannot win the weak by sharing our wealth with them. They feel our generosity as oppression. St. Vincent De Paul cautioned his disciples to deport themselves so that the poor "will forgive them the bread you give them."

Source: In Watermelon Sugar

Editorial in Analog Science Fiction/Fact magazine (1970)

Slightly edited version of text in Men and Women of the Corporation, Basic Books, 1977

Letter http://oll.libertyfund.org/index.php?option=com_content&task=view&id=1407&Itemid=283 to Mandell Creighton (5 April 1887), published in Historical Essays and Studies, by John Emerich Edward Dalberg-Acton (1907), edited by John Neville Figgis and Reginald Vere Laurence, Appendix, p. 504; also in Essays on Freedom and Power (1972)
Paraphrased variant: All power tends to corrupt and absolute power corrupts absolutely.
Context: I cannot accept your canon that we are to judge Pope and King unlike other men, with a favorable presumption that they did no wrong. If there is any presumption it is the other way against holders of power, increasing as the power increases. Historic responsibility has to make up for the want of legal responsibility. Power tends to corrupt and absolute power corrupts absolutely. Great men are almost always bad men, even when they exercise influence and not authority: still more when you superadd the tendency or the certainty of corruption by authority. There is no worse heresy than that the office sanctifies the holder of it. That is the point at which the negation of Catholicism and the negation of Liberalism meet and keep high festival, and the end learns to justify the means.

The Corruptions of Our Time, p. 248
The Corrupt Society - From Ancient Greece To Present-Day America (1975)