Under a system of perfectly free commerce, each country naturally devotes its capital and labour to such employments as are most beneficial to each. This pursuit of individual advantage is admirably connected with the with the universal good of the whole. By stimulating industry, by rewarding ingenuity, and by using most efficaciously the peculiar powers bestowed by nature, it distributes labour most effectively and most economically: while, by increasing the general mass of productions, it diffuses general benefit, and binds together, by one common tie of interest and intercourse, the universal society of nations throughout the civilized world.
Chapter VII, On Foreign Trade, p. 81 (See also.. Karl Marx, Das Kapital,(Buch II), Chapter XX, p. 474) - The Principles of Political Economy and Taxation (1821) (Third Edition)