The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane.

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

(Spurious)
    (Source)

The earliest identifiable citation appears as an epigraph to Leo Tolstoy, Bethink Yourselves!, ch. 8 (1904), in Recollections & Essays [tr. Aylmer Maud (1937)]. A cleaner copy can be found at the Nonresistance.org site. The classic (and presumably abridged) version of Bethink Yourselves!, as translated by Chertkov (1904), does not include any of the copious epigraphs, including this one.

Regardless, this phrase is not clearly found in Marcus Aurelius' Meditations, though other parts of the lengthy epigraph appear to be. If ordered by position, this would presumably fall somewhere from 10.6 to 10.8 (i.e., the preceding and following sentences are more clearly identifiable), but the language of this sentence does not seem to line up.