If an idiot were to tell you the same story every day for a year, you would end by believing it.

Edmund Burke (1729-1797) Anglo-Irish statesman, orator, philosopher

(Attributed)
    (Source)

Quoted without citation in John Frederick Boyes, Lacon in Council, "Literature, Poetry, Oratory, Genius, &c." (1865). That is the earliest reference I could find for this quote.

Sometimes misattributed to Horace Mann.