History of the Peloponnesian War
Thucydides
Thucydides. The history of the Peloponnesian War, Volume 1-2. Dale, Henry, translator. London: Heinemann and Henry G. Bohn, 1851-1852.
The Athenians were also joined, in accordance with the treaty, by a thousand horse of the Thessalians, who went over during the action to the Lacedaemonians.
A battle having been fought at Tanagra in Boeotia, the Lacedaemonians and their allies were victorious, and there was much bloodshed on both sides.