History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
For these reasons the Lacedaemonians were friendly towards him, and it was not least on that account that he trusted in Gylippus and surrendered himself to him. But it was said that some of the Syracusans were afraid, seeing that they had been in communication with him, lest, if he were subjected to torture on that account, he might make trouble for them in the midst of their success; and others, especially the Corinthians, were afraid, lest, as he was wealthy,[*](He was worth 100 talents, according to Lysias, xix. 47. His property was chiefly in silver mines. He employed 1000 slaves in the mines at Laurium (Xen., De Vect. iv. 14).) he might by means of bribes make his escape and cause them fresh difficulties; they therefore persuaded their allies and put him to death.
For this reason, then, or for a reason very near to this, Nicias was put to death—a man who, of all the Hellenes of my time, least deserved to meet with such a calamity, because of his course of life that had been wholly regulated in accordance with virtue.