History of the Peloponnesian War

Phrynichus now saw that there would be a proposal to restore Alcibiades, and that the Athenians would consent to it; and fearing after what he had said against it that Alcibiades, if restored, would revenge himself upon him for his opposition, had recourse to the following expedient. He sent a secret letter to the Lacedaemonian admiral,

Astyochus, who was still in the neighbourhood of Miletus, to tell him that Alcibiades was ruining their cause by making Tissaphernes the friend of the Athenians, and containing an express revelation of the rest of the intrigue, desiring to be excused if he sought to harm his enemy even at the expense of the interests of his country.

However, Astyochus, instead of thinking of punishing Alcibiades, who, besides, no longer ventured within his reach as formerly, went up to him and Tissaphernes at Magnesia, communicated to them the letter from Samos, and turned informer, and if report may be trusted, became the paid creature of Tissaphernes, undertaking to inform him as to this and all other matters; which was also the reason why he did not remonstrate more strongly against the pay not being given in full.

Upon this Alcibiades instantly sent to the authorities at Samos a letter against Phrynichus, stating what he had done, and requiring that he should be put to death.