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.

After this the conflict commenced, the Argives and their allies advancing eagerly and impetuously, but the Lacedaemonians slowly and to the music of many flute-players placed among them according to custom, not with any religious motive, but in order that they might march up with even step and keeping time without breaking their order, as large armies are apt to do in going into battle.