Alcibiades (25.8-25.9)

But Phrynichus knew all the while that he would do so, and expected a second denunciation from Alcibiades. So he got the start of him by telling the Athenians himself that the enemy were going to attack them, and advising them to have their ships manned and their camp fortified.

The Athenians were busy doing this when again a letter came from Alcibiades bidding them beware of Phrynichus, since he had offered to betray their fleet to the enemy. This letter they disbelieved at the time, supposing that Alcibiades, who must know perfectly the equipment and purposes of the enemy, had used his knowledge in order to calumniate Phrynichus falsely.