It was one Sunday evening early in September of the year 1903 that I received one of Holmes’s laconic messages: “Come at once if convenient — if inconvenient come all the same. S.H.'”

Arthur Conan Doyle (1859-1930) British writer and physician
“The Adventure of the Creeping Man,” Strand Magazine (Mar 1923)
    (Source)

Reprinted in The Case-Book of Sherlock Holmes (1927).