An historian is an unsuccessful novelist.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 7, § 21 (1916)
    (Source)

Variants:

HISTORIAN. An unsuccessful novelist.
[A Book of Burlesques, "The Jazz Webster" (1924)]

Historian - An unsuccessful novelist.
[Chrestomathy, ch. 30 "Sententiae" (1949)]