… is from page 127 of Thomas Sowell’s 2008 book, Economic Facts and Fallacies (original emphasis; footnote omitted):
There was a time when it was meaningful to speak of “the idle rich” and the “toiling poor” but that time has long past [sic]. Most households in the bottom 20 percent by income do not have any full-time, year-round worker and 56 percent of these households do not have anyone working even part-time. Some of the low-income households contain single mothers on welfare and their children. Some such households consist of retirees living on Social Security or others who are not working, or who are working sporadically or part-time, because of disabilities or for other reasons.
Household income data can therefore be very misleading, whether comparing income differences as of a given time or following changes in income over years.
The fact that data on income are treacherous to interpret is one that cannot be emphasized too strongly. See also, for example, this recent post by James Pethokoukis.