Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

86. The Syracusans and their allies collected their forces and returned with the spoil, and as¹ many prisoners as they could take with them, into the city. [2] The captive Athenians and allies they deposited in the quarries, which they thought would be the safest place of confinement. Nicias and Demosthenes they put to the sword, although against the will of Gylippus. For Gylippus thought that to carry home with him to Lacedaemon the generals of the enemy, over and above all his other successes, would be a brilliant triumph. [3] One of them, Demosthenes, happened to be the greatest foe, and the other the greatest friend of the Lacedaemonians, both in the same matter of Pylos and Sphacteria. For Nicias had taken up their cause², and had persuaded the Athenians to make the peace which set at liberty the prisoners taken in the island. [4] The Lacedaemonians were grateful to him for the service, and this was the main reason why he trusted Gylippus and surrendered himself to him. But certain Syracusans, who had been in communication with him, were afraid (such was the report) that on some suspicion of their guilt he might be put to the torture and bring trouble on them in the hour of their prosperity. Others, and especially the Corinthians, feared that, being rich, he might by bribery escape and do them further mischief. [5] So the Syracusans gained the consent of the allies and had him executed. For these or the like reasons he suffered death. No one of the Hellenes in my time was less deserving of so miserable an end; for he lived in the practice of every virtue.