Titus Livius (Livy), The History of Rome, Book 21
Rev. Canon Roberts, Ed.

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Between the two armies there was a stream with very high banks which were overgrown with marshy grass and the brambles and brushwood which are generally found on waste ground. After riding round the place and satisfying himself from personal observation that it was capable of concealing even cavalry, Hannibal, turning to his brother Mago, said, "This will be the place for you to occupy. [2] Pick out of our whole force of cavalry and infantry a hundred men from each arm, and bring them to me at the first watch, now it is time for food and rest." He then dismissed his staff. Presently Mago appeared with his 200 picked men. [3] "I see here," said Hannibal, "the very flower of my army, but you must be strong in numbers as well as in courage. Each of you therefore go and choose nine others like himself, from the squadrons and the maniples. Mago will show you the place where you are to lie in ambuscade, you have an enemy who are blindly ignorant of these practices in war." [4] After sending Mago with his 1000 infantry and 1000 cavalry to take up his position, Hannibal gave orders for the Numidian cavalry to cross the Trebia in the early dawn and ride up to the gates of the Roman camp; then they were to discharge their missiles on the outposts and so goad the enemy on to battle. When the fighting had once started they were gradually to give ground and draw their pursuers to their own side of the river. [5] These were the instructions to the Numidians; the other commanders, both infantry and cavalry, were ordered to see that all their men had breakfast, after which they were to wait for the signal, the men fully armed, the horses saddled and ready. [6] Eager for battle, and having already made up his mind to fight, Sempronius led out the whole of his cavalry to meet the Numidian attack, for it was in his cavalry that he placed most confidence; [7] these were followed by 6000 infantry and at last the whole of his force marched on to the field. It happened to be the season of winter, a snowstorm was raging, and the district, situated between the Alps and the Apennines, was rendered especially cold by the vicinity of rivers and marshes. [8] To make matters worse, men and horses alike had been hurriedly sent forward, without any food, without any protection against the cold, so they had no heat in them and the chilling blasts from the river made the cold still more severe as they approached it in their pursuit of the Numidians. [9] But when they entered the water which had been swollen by the night's rain and was then breast high, their limbs became stiff with cold, and when they emerged on the other side they had hardly strength to hold their weapons; they began to grow faint from fatigue and as the day wore on, from hunger.