History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
fearing that the Athenians might leave quietly during the night and get through the most difficult places before they could prevent this, he sent certain of his own friends with some horsemen to the Athenian camp when it was growing dark. These rode up close enough to be heard and called upon certain persons by name, as though they were friends of the Athenians—for there were some who regularly reported to Nicias all that went on in Syracuse[*](cf. 7.48.2.)—and bade them tell Nicias not to lead his army away that night, since the Syracusans were guarding the roads, but to withdraw at his leisure, in the daytime, after having made full preparations.
After saying this these men departed, and those who had heard reported it to the Athenian generals.