Given this reference text, what was John Dalberg-Acton's most famous pronouncement, and to whom did he make it?

In a letter to Mandell Creighton, John Dalberg-Acton made his most famous pronouncement: "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."