Always do right. This will gratify some people, and astonish the rest.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Letter (1901-02-16) to the Young People’s Society, Greenpoint Presbyterian Church, Brooklyn
    (Source)

The Society had invited Twain to come and speak. In response, he sent a card with this aphorism on it. A reproduction is in the frontispiece of Bernard DeVoto, ed., Mark Twain in Eruption (1922), which was the earliest citation I could find.