No evidence that this phrasing is due to Stalin, and it does not appear in English translations of his philosphical works. Earliest English is found in 1979 in US defense industry, presumably defense consultant Thomas A. Callaghan Jr. The connection of sufficient quantitative change leading to qualitative change is found in Marxist philosophy, by Marx and Engels, drawing from Hegelian philosophy and Ancient Greek philosophy. Marx and Engels are quoted by Stalin, but this formulation appears to be a modern American form; see quantity for details.
Stalin may have said that way before World War II, there is evidence in his Russian-language books, for example here http://www.modernlib.ru/books/stalin_iosif_vissarionovich/tom_14/read_16/.
Variante: Quantity is quality.
Quelle: Re: "Quantity has a quality all its own" source? http://h-net.msu.edu/cgi-bin/logbrowse.pl?trx=vx&list=h-russia&month=1004&week=a&msg=ljEwsM4dMrpmUGVfI7EGqg, Tim Davenport, h-russia https://networks.h-net.org/h-russia, April 5, 2010