
Orot Yisrael, Ch. 5, article 10, p. 156; as quoted in "The Distinction between Jews and Gentiles in Torah" by Rabbi David Bar Chaim http://www.daatemet.org.il/articles/article.cfm?article_id=119&lang=en
Variant:
The dissimilarity between the Jewish soul, in all its independence, inner desires, longings, character and standing vis-à-vis the soul of all the Gentiles — on all of their levels — is greater and deeper than the difference between the soul of a man and the soul of an animal, for the difference in the latter case is one of quantity, while the difference in the first case is one of essential quality
As quoted in "A British Synagogue Bans a Famous Hassidic Text!" (February 2010) by Rabbi Michael Leo Samuel http://rabbimichaelsamuel.com/2010/02/2744/#_ftn1.
Orot