“Be respectful to your superiors, if you have any.”
"Advice to Youth", speech to The Saturday Morning Club, Boston, 15 April 1882. Mark Twain Speaking (1976), ed. Paul Fatout, p. 169 http://books.google.com/books?id=mkFgXWvUkVoC&pg=PA169
Variant: Be respectful to your superiors, if you have any.
Source: The Wit and Wisdom of Mark Twain