History of the Peloponnesian War

But when Phrynichus saw that there would be a proposal for the recall of Alcibiades, and that the Athenians would accede to it, being afraid, on considering the opposite tendency of what had been maintained by himself, that if he were restored he would do him some mischief, as one who had impeded his plans, he had recourse to the following device.

He sent to Astyochus the Lacedaemonian admiral, who was still in the neighbourhood of Miletus, with secret instructions that Alcibiades was ruining their cause by bringing Tissaphernes into friendship with the Athenians; expressly mentioning all the other matters also, and pleading that it was pardonable in him to devise evil against a man who was an enemy, even though it were to the detriment of the state.

Now Astyochus did not so much as think of punishing Alcibiades—especially as he no longer put himself in his power as he used to do—but having gone up to him and Tissaphernes at Magnesia, at once told them the contents of the letter from Samos, acting as an informer to them, and for his own private gain devoting himself, as was said, to the interest of Tissaphernes both on these and on all other matters: