Who said "a common enemy unites the people"?

Question

I'm looking for concrete evidence. I've heard that Adolf Hitler himself is credited with the quote, but I need a source.

Thanks.

Answer 1

The earliest usage I found was in The Letters to the Seven Churches of Asia by W. M. Ramsay in 1904.
Chapter 19: Smyrna: The City of Life
"A common danger and a common enemy united them"

Answer 2

I believe it was an earlier German by the name of Otto von Bismark who said it, as a justification to make war on the French in order to bring all of the little southern German city states under Prussian rule in the late 1800s. The strategy worked except for the one holdout, Lichtenstein, which remains a small city state to this day.