A great man shows his greatness by the way he treats little men.

Thomas Carlyle (1795-1881) Scottish essayist and historian

(Spurious)
    (Source)

Attributed with this wording to Carlyle by Dale Carnegie in How to Win Friends And Influence People, ch. 1 (1936). Though Carlyle wrote numerous times on character and greatness, this phrase (or variants of it) cannot be found in Carlyle's known works.

The earliest attribution of something like this to Carlyle comes from a sermon by John Timothy Stone at the Northfield Young Women's Conference (1908-07-07): "You can discover a great man, or see a great man, by the way he treats little men."

It subsequently shows up, variously, mostly in religious contexts, e.g.:

• "A great man shows his greatness by manner in which he treats little men." [Unattributed (1913)]
• "The measure of a great man is the way in which he treats little men." [George E. Mayer (1916-05-25)]

John Timothy Stone (as above) published Christianity in Action (1930), six years before Carnegie's book. In ch. 62 "Friends as You Go," he used the same phrasing Carnegie did six years later, again attributing it to Carlyle. It's been suggested Carnegie drew the quotation from Stone's book.

See also Eldridge (1948), which discusses numerous parallels to this sentiment.