History of the Peloponnesian War

And when Phrynichus learned betimes that Astyochus was working to injure him and that a letter from Alcibiades about these matters had all but come, he anticipated it by himself informing the army that the enemy intended, seeing that Samos had no walls and that not all the ships were anchored inside the harbour, to attack the camp; he said that he had certain information of this, and that they ought to fortify Samos as quickly as possible and keep a watch upon everything. Now he was general, and was within his powers in acting thus on his own authority.

So they set about preparing the fortifications, and in consequence of this Samos, which in any case would soon have been ready, was more quickly fortified. Not long afterward came the letter from Alcibiades, saying that the army was being betrayed by Phrynichus and that the enemy would soon attack.

But since Alcibiades was not regarded as a trustworthy man, but was believed, as he knew beforehand the plans of the enemy, to be actuated by personal enmity in attributing to Phrynichus a guilty knowledge of them, the accusation did Phrynichus no harm, but rather confirmed his statement by giving the same information.