History of the Peloponnesian War
Thucydides
Thucydides. The history of the Peloponnesian War, Volume 1-2. Dale, Henry, translator. London: Heinemann and Henry G. Bohn, 1851-1852.
And now Gylippus went away into the rest of Sicily for forces, intending to raise them both for sea and land service; and at the same time to bring over any of the cities that was either not hearty in the cause, or had hitherto stood entirely aloof from the war.
Other ambassadors of the Syracusans and Corinthians were also sent to Lacedaemon and Corinth, in order that a fresh force might be sent over to them, in whatever way might answer best, whether in merchantmen, or boats, or any how else; since the Athenians too were sending for reinforcements.
Moreover, the Syracusans proceeded to man and practise a fleet, for the purpose of making an attempt in that way also, and were in other respects much more full of confidence.
Nicias observing this, and seeing that the enemy's strength and their own difficulties were every day increasing, sent, on his side also, to Athens; having, indeed, on many other occasions sent messengers at the time of their several operations, but doing so then especially, because he thought that they were in a perilous condition, and that unless those at home recalled them as quickly as possible, or sent out no small reinforcement, there was no hope of preservation for them.