“Genius” (which means transcendent capacity of taking trouble, first of all).

Thomas Carlyle (1795-1881) Scottish essayist and historian

History of Friedrich II of Prussia, called Frederick the Great, Book 4, ch. 3 (1858–1865)
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Usually misquoted as "Genius is an infinite capacity for taking pains," which is sometimes misattributed to Samuel Johnson.

See Doyle (1886).

See also more discussion here about the quote.