The History of the Grecian War (1.91.2-1.91.4)

Themistocles, when he saw this, wished them not to be led by reports, but rather to send thither some of their own, such as were honest men, and, having informed themselves, would relate the truth, which they also did.

And Themistocles sendeth privily to the Athenians about the same men to take order for their stay with as little appearance of it as they could and not to dismiss them till their own ambassadors were returned (for by this time were arrived those that were joined with him, namely, Abronychus the son of Lysicles, and Aristides the son of Lysimachus, and brought him word that the wall was of a sufficient height);

for he feared lest the Lacedaemonians, when they knew the truth, would refuse to let them go. The Athenians therefore kept there those ambassadors according as it was written to them to do. Themistocles, coming now to his audience before the Lacedaemonians, said plainly, that the city of Athens was already walled, and that sufficiently for the defence of those within, and that if it shall please the Lacedaemonians upon any occasion to send ambassadors unto them, they were to send thenceforward as to men that understood what conduced both to their own and also to the common good of all Greece.