“They say that genius is an infinite capacity for taking pains,” he [Holmes] remarked with a smile. “It’s a very bad definition, but it does apply to detective work.”

Arthur Conan Doyle (1859-1930) British writer and physician

Story (1886-04), “A Study in Scarlet,” Part 1, ch. 3, Beeton’s Christmas Annual, Vol. 28 (1887-11-21)
    (Source)

Published in novel form 1888-07.

The quote Holmes refers to is usually attributed to Scottish historian and essayist Thomas Carlyle, but is a misquote of what he says on the subject. Interestingly, Holmes, in the same story, earlier claims not to know Carlyle's works, though he here supposedly quotes him.