I suspect it is his contribution to the study of language that gets him
cited, not his political views.
Regarding the source, I've found it in numerous places. A quick search on
altavista (+"most cited" +humanities +Chomsky) turns up this extract from
"Chomsky is one of this century's most important figures, and has been
described as one who will be for future generations what Galileo, Descart=
Newton, Mozart, or Picasso have been for ours. He is the most cited livin=
person =AD four thousand citations of his work are listed in the Arts and
Humanities Citation Index for the years 1980 through 1992 =AD and eighth =
shortlist, which includes the likes of Marx and Freud, of the most cited
figures of all time. Chomsky is also a vital point of reference in the
sciences; from 1974 to 1992 he was cited 1,619 times, according to the
Science Citation Index. Among the innumerable honors he has been awarded =
the 1988 Kyoto Prize, described as the Japanese equivalent of the Nobel
Prize, for his contribution to basic sciences."
Footnote 2 reads:
"The first ten entries on the list are, in order: Marx, Lenin, Shakespear=
Aristotle, the Bible, Plato, Freud, Chomsky, Hegel, and Cicero."