The History of the Grecian War (1.101.2-1.102.1)

which, unknown to the Athenians, they promised to do and also had done it, but by an earthquake that then happened, they were hindered. In which earthquake their Helots, and of neighboring towns the Thuriatae and Aethaeans, revolted and seized on Ithome. Most of these Helots were the posterity of the ancient Messenians brought into servitude in former times, whereby also it came to pass that they were called all Messenians.

Against these had the Lacedaemonians now a war at Ithome. The Thasians in the third year of the siege rendered themselves to the Athenians upon condition to raze their walls, to deliver up their galleys, to pay both the money behind and for the future as much as they were wont, and to quit both the mines and the continent.

The Lacedaemonians, when the war against those in Ithome grew long, amongst other their confederates sent for aid to the Athenians, who also came with no small forces under the command of Cimon.