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.
During the same summer also the Dians took Thyssus, a town on the promontory of Athos, which was in alliance with the Athenians.
All this summer there was intercourse between the Athenians and Lacedaemonians, but both parties began to suspect one another directly after the conclusion of the treaty, owing to their failure to give back to one another the places specified.
The Lacedaemonians, though they had drawn the lot to make restoration first, had not restored Amphipolis and the other places; nor had they made their allies in Thrace accept the treaty, nor the Boeotians, nor the Corinthians, though they continually professed that they would join the Athenians in coercing these states, if they were unwilling; and they proposed dates, without making a written agreement, on which those who did not accede to the treaty were to be enemies of both.
Seeing, then, that none of these things was actually being done, the Athenians suspected the Lacedaemonians of having no just intentions, and so not only did not restore Pylos when the Lacedaemonians demanded it, but even repented that they had restored the prisoners taken on the island, and they continued to hold the other places, waiting until the Lacedaemonians should have fulfilled their part of the contract.
The Lacedaemonians said that they had done what was possible; for they had restored the prisoners of the Athenians that were in their hands, had brought back their troops in Thrace, and had done whatever else had been in their power. As to Amphipolis, however, they said that they were not in control of it, so as to deliver it up;