History of the Peloponnesian War

As for all the other prisoners of the Athenians and their confederates, they put them into the quarries as the safest custody. But Nicias and Demosthenes they killed, against Gylippus' will. For Gylippus thought the victory would be very honourable if, over and above all his other success, he could carry home both the generals of the enemy to Lacedaemon.

And it fell out that one of them, Demosthenes, was their greatest enemy for the things he had done in the island and at Pylus; and the other, upon the same occasion, their greatest friend. For Nicias had earnestly laboured to have those prisoners which were taken in the island to be set at liberty by persuading the Athenians to the peace.