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.
But again Nicias urged him to go and offered to resign his command of the expedition against Pylos, calling the Athenians to witness that he did so. And the more Cleon tried to evade the expedition and to back out of his own proposal, the more insistently the Athenians, as is the way with a crowd, urged Nicias to give up the command and shouted to Cleon to sail.
And so, not knowing how he could any longer escape from his own proposal, he undertook the expedition, and, coming forward, said that he was not afraid of the Lacedaemonians, and that he would sail without taking a single Athenian soldier, but only the Lemnian and Imbrian troops which were in Athens and a body of targeteers which had come from Aenos, and four hundred archers from other places. With these, in addition to the troops now at Pylos, he said that within twenty days he would either bring back the Lacedaemonians alive or slay them on the spot.
At this vain talk of his there was a burst of laughter on the part of the Athenians, but nevertheless the sensible men among them were glad, for they reflected that they were bound to obtain one of two good things—either they would get rid of Cleon, which they preferred, or if they were disappointed in this, he would subdue the Lacedaemonians for them.
When he had arranged everything in the assembly and the Athenians had voted in favour of his expedition, he chose as his colleague Demosthenes, one of the generals at Pylos, and made haste to set sail.
He selected Demosthenes because he had heard that he was planning to make his landing on the island. For his soldiers, who were suffering because of the discomforts of their position, where they were rather besieged than besiegers, were eager to run all risks. And Demosthenes himself had also been emboldened by a conflagration which had swept the island.