Recollections of the Table-Talk of Samuel Rogers, to which is added Porsoniana
(London: Robson, Levey, and Franklyn, 1856), p. 323 (on Richard Porson):
He confessed to me and the present Bishop of Durham (Maltby), that he knew comparatively little of Thucydides,—that, when he read him, he was obliged to mark with a pencil, in almost every page, passages which he did not understand.
I can't find the source of Benjamin Jowett's definition of a scholar, mentioned by Hugh Lloyd-Jones, Blood for the Ghosts: Classical Influences in the Nineteenth and Twentieth Centuries
(1982; rpt. Baltimore: The Johns Hopkins University Press, 1983), p. 158:
Jowett defined a scholar as a man who read Thucydides with his feet on the mantlepiece; by that test he was a scholar, scarcely by any other.