“Tis strange,-but true; for truth is always strange;
Stranger than fiction: if it could be told,
How much would novels gain by the exchange!
How differently the world would men behold!”

Or, for that matter, as true.
Source: The Areas of My Expertise (2005), p. 18

1910s
Source: A Little Book in C Major (1916)

Scraps and Morsels (1960).
Context: Strange reading? It is meant to be. The world is full of romantic, macabre, improbable things which would never do in works of fiction. When those that come within one man's notice are gathered together in a scrapbook, they tell of a world which sobersided folk may not choose to recognize as their own. But it is their own; I have the evidence.

The Monthly Magazine

A System of Magick (1726).

Source: Earthsea Books, The Other Wind (2001), Chapter 2 “Palaces” (p. 72)

Le seul véritable voyage, le seul bain de Jouvence, ce ne serait pas d'aller vers de nouveaux paysages, mais d'avoir d'autres yeux, de voir l'univers avec les yeux d'un autre, de cent autres, de voir les cent univers que chacun d'eux voit, que chacun d'eux est.
Source: In Search of Lost Time, Remembrance of Things Past (1913-1927), Vol. V: The Captive (1923), Ch. II: "The Verdurins Quarrel with M. de Charlus"

Pudd'nhead Wilson's New Calendar, Ch. XV
Misquoted as "Why shouldn’t truth be stranger than fiction? Fiction, after all, has to make sense." by Laurence J. Peter in "Peter’s Quotations: Ideas for Our Time", among many others.
Following the Equator (1897)
Source: Following the Equator: A Journey Around the World

The Complete Neurotic's Notebook (1981), Unclassified