.......... 
I have been looking for this quote for years!
Caesar, in writing home, said of the Britons, “They are the most ignorant people I have ever conquered. They cannot be taught music.” Cicero, in writing to his friend Atticus, advised him not to buy slaves in England, “because,” said he, “they cannot be taught to read, and are the ugliest and most stupid race I ever saw.”
William Wells Brown (1863), The Black Man: His Antecedents, His Genius, and His Achievements (Boston: James Redpath), pp. 33-4; quoted on p. 92 by Mia Bay (2000), The White Image in the Black Mind: African American Ideas About White People, 1830-1925 (Oxford: Oxford University Press: 019510045X).
Now where in Caesar's Commentaries and Cicero's Letters to Atticus are the originals?
UPDATE: The internet to the rescue (actually the UVA Classics Department)! Gregory Hays:
I think one of the quotations in Brown is a very vague paraphrase of Cicero, Letters to Atticus 4.16.7. Here's D.R. Shackleton Bailey's English rendering:
... The Paccius letter having been answered, let me tell you the rest of my news. A letter from my brother contains some quite extraordinary things about Caesar's warm feelings towards me, and is corroborated by a very copious letter from Caesar himself. The result of the war against Britain is eagerly awaited, for the approaches to the island are known to be 'warded with wondrous massy walls.' It is also now ascertained that there isn't a grain of silver on the island nor any prospect of booty apart from captives, and I fancy you won't expect any of them to be highly qualified in literature or music!
Best,
Greg Hays
Ah. There appears to be some confusion in the text--does the passage follow "Paccianae epistulae respondi" (I have responded to the letter brought by Paccius) in 4.16 (Scr. Romae ex. m. Iun. aut in. Quint. a. 700) or does it follow "in illis quidem tribus libris, quos tu dilaudas, nihil reperio" (no report in those three books which you praise) in 4.17 (Scr. Romae K. Oct a. 700)?
I am, Zeus Pater knows, no Classics scholar. But from context--we are talking about profit for the res publica from imperialism here--I would be more inclined to translate "litteris aut musicis eruditos" as something like "taught to read or play" rather than Shackleton Bailey's "highly qualified in literature or music." The point appears to be that Caesar is engaged in folly: raiding an island where (a) there is no silver to be stolen, and (b) the slaves he will capture won't be worth much.
By what process Brown gets "ugliest and most stupid race" I do not know...
