… is from page 23 of William J. Bernstein’s 2004 book, The Birth of Plenty: How the Prosperity of the Modern World Was Created (original emphasis):
[E]ven the most wildly optimistic estimates suggest no more than a doubling or tripling in global GDP between the year A.D. 1 and A.D. 1000, versus the eightfold increase in the 172 years following 1820. During the same 172-year period, per capita GDP in the U.K. grew tenfold; in the U.S., twentyfold.