Pausanias Description of Greece 10.1.11

[11]
  Afterwards the Phocians discovered a stratagem quite as clever as their former ones. For when the armies were lying opposite each other at the pass into Phocis , five hundred picked men of Phocis , waiting until the moon was full, attacked the Thessalians on that night, first smearing themselves with chalk and, in addition to the chalk, putting on white armour. It is said that there then occurred a wholesale slaughter of the Thessalians, who thought this apparition of the night to be too unearthly to be an attack of their enemies. It was Tellias of Elis who devised this stratagem also for the Phocians to use against the Thessalians.

[11] After this, the men of Phokis came up with a stratagem quite as clever as their former ones. When the two armies were encamped opposite each other at the pass into Phokis, five hundred picked men of Phokis, waiting until the moon was full, attacked the Thessalians on that night, first smearing themselves with gypsum [ gupsos] and, in addition to the gypsum [ gupsos], putting on white armor [ leuka]. It is said that there then occurred a wholesale slaughter of the Thessalians, who thought this apparition of the night to be too superhuman [ theion] to be an attack of their enemies. It was Tellias of Elis who devised this stratagem as well for the men of Phokis to use against the Thessalians.