Aubrey's Brief Lives
. Edited from the Original Manuscripts and with an Introduction by Oliver Lawson Dick, 2nd ed. (London: Secker and Warburg, 1950), pp. xcv-xcvi (Dick quoting Aubrey):
Mr. Hobbes told me, that G. Duke of Buckingham had at Paris when he was about twenty yeares old, desired Him to reade Geometrie to him: his Grace had great naturall parts, and quicknesse of witt; Mr. Hobbes read, and his Grace did not apprehend, which Mr. Hobbes wondered at: at last, Mr. Hobbes observed that his Grace was at mastrupation (his hand in his Codpiece).
"Mr. Hobbes" is of course Thomas Hobbes; "G. Duke of Buckingham" is George Villiers, 2nd Duke of Buckingham.