“When I discovered that history is a fable, and a poor one at that, I started looking for a better fable, and found theology.”

Conversation with Emmanuel, comte de Las Cases (20 November 1816), Mémorial de Sainte Hélène, v. 4, p. 251 http://books.google.com/books?id=945jAAAAMAAJ&vq=%22fable%20agreed%20upon%22&pg=PA251. However, the phrase predates Napoleon. Helvétius attributes it to Bernard le Bovier de Fontenelle, De l'esprit (1758), p. 443 http://books.google.com/books?id=N7g8AAAAcAAJ&vq=%22fable%20convenue%22&pg=RA1-PA443

Quoted in [1906, Six Historic Americans, John E., Remsburg, chapter 2, New York, The Truth Seeker Company, 13504056M, 2219498, 74, http://www.archive.org/details/sixhistoricameri00rems], who claimed it to be from a letter to "Dr. Woods." The full letter is never reproduced, and the Jefferson Foundation lists http://www.monticello.org/site/jefferson/superstition-christianity-quotation the quotation as spurious.
Disputed

Patheos, Philosophistry http://www.patheos.com/blogs/reasonadvocates/2017/04/12/philosophistry/ (April 12, 2017)

Joseph Fourier, p. 408.
Biographies of Distinguished Scientific Men (1859)
Context: The ancients had a taste, let us say rather a passion, for the marvellous, which caused them to forget even the sacred duties of gratitude. Observe them, for example, grouping together the lofty deeds of a great number of heroes, whose names they have not even deigned to preserve, and investing the single personage of Hercules with them. The lapse of ages has not rendered us wiser in this respect. In our own time the public delight in blending fable with history. In every career of life, in the pursuit of science especially, they enjoy a pleasure in creating Herculeses.

Miscellaneous

Gesta Regum Anglorum (1125), quoted in M. J. Cohen and John Major (eds.), History in Quotations (Cassell, 2004), p. 160

Source: Xenocide

A Week on the Concord and Merrimack Rivers http://www.gutenberg.org/dirs/etext03/7cncd10.txt (1849), Sunday
Context: The hidden significance of these fables which is sometimes thought to have been detected, the ethics running parallel to the poetry and history, are not so remarkable as the readiness with which they may be made to express a variety of truths. As if they were the skeletons of still older and more universal truths than any whose flesh and blood they are for the time made to wear. It is like striving to make the sun, or the wind, or the sea symbols to signify exclusively the particular thoughts of our day. But what signifies it? In the mythus a superhuman intelligence uses the unconscious thoughts and dreams of men as its hieroglyphics to address men unborn. In the history of the human mind, these glowing and ruddy fables precede the noonday thoughts of men, as Aurora the sun's rays. The matutine intellect of the poet, keeping in advance of the glare of philosophy, always dwells in this auroral atmosphere.