Plutarch, Aristides
Bernadotte Perrin, Ed.

After this cavalry battle, both sides refrained from further fighting for a long time, since only as they acted on the defensive would victory be theirs—so the soothsayers interpreted the sacrifices alike for Persians and Hellenes,—but if they attacked, defeat. At last Mardonius, since he had supplies remaining for only a few days, and since the Hellenes were ever increasing in number as fresh bodies joined them, impatiently determined to wait no longer, but to cross the Asopus at daybreak and attack the Athenians unexpectedly. During the evening he gave the watchword to his commanders. [2]

But about midnight a solitary horseman quietly approached the camp of the Hellenes, and falling in with the outposts, ordered that Aristides the Athenian come to him. He was speedily obeyed, and then said: ‘I am Alexander the Macedonian, and I am come at the greatest peril to myself, out of my good-will toward you, that no suddenness of attack may frighten you into inferior fighting. [3] Mardonius will surely give battle on the morrow, not because he has substantial hope or even courage, but because he is destitute of provisions. His soothsayers, indeed, are trying to keep him from battle by unpropitious sacrifices and oracular utterances, while his army is full of dejection and consternation; but he must needs boldly try his fortune, or sit still and endure extremest destitution.’ [4] When he had told him this, Alexander begged Aristides to keep the knowledge to himself and bear it well in mind, but to tell it to none other. Aristides replied that it was not honorable to conceal this knowledge from Pausanias, since it was on him that the supreme command devolved, but that it should not be told the other leaders before the battle; though in case Hellas were victorious, no man should remain ignorant of Alexander's zeal and valor. [5] After this conversation, the king of the Macedonians rode off back again, and Aristides went to the tent of Pausanias and told him all that had been said. Then they summoned the other leaders and gave them orders to keep the army in array, since there was to be a battle.