Xenophon (c.430–354 BCE) was a native of Athens who wrote about contemporary events and people. He was also a soldier and mercenary. These excerpts are taken from his account of the war between Sparta and Thebes in 369-8 BCE, during which Dionysius of Syracuse sent Celtic mercenaries to fight on the side of Sparta.
The following is from Xenophon in Seven Volumes, trans. Carleton L. Brownson. Harvard University Press, Cambridge:1918-1921.
§ 7.1.20. Just after these events had happened, the expedition sent by Dionysius to aid the Lacedaemonians sailed in, numbering more than twenty triremes [war galleys]. And they brought Celts, Iberians, and about fifty horsemen. On the following day the Thebans and their allies, after forming themselves in detached bodies and filling the plain as far as the sea and as far as the hills adjoining the city, destroyed whatever of value there was in the plain. And the horsemen of the Athenians and of the Corinthians did not approach very near their army, seeing that the enemy were strong and numerous.
§ 7.1.31. When these words had been spoken, it is said that from a clear sky there came lightnings and thunderings of favourable omen for him; and it chanced also that on the right wing was a sanctuary and a statue of Heracles [the ancestor of the kings of Sparta]. As a result, therefore, of all these things, it is reported that the soldiers were inspired with so much strength and courage that it was a task for their leaders to restrain them as they pushed forward to the front. And when Archidamus led the advance, only a few of the enemy waited till his men came within spear-thrust; these were killed, and the rest were cut down as they fled, many by the horsemen and many by the Celts.