Titus Livius (Livy), The History of Rome, Book 27
Cyrus Evans, Ed.

41. Hannibal, having drawn together his forces from all quarters, both those which he had in winter quarters, and those which he had in the garrisons of the Bruttian territory, came to Grumentum in Lucania, with the hope of regaining the towns which through fear had revolted to the Romans. [2] To the same place the Roman consul proceeded from Venusia, exploring the way as he went, and pitched his camp about fifteen hundred paces from the enemy. [3] The rampart of the Carthaginians seemed almost united with the walls of Grumentum, though five hundred paces intervened. [4] Between the Carthaginian and Roman camps lay a plain; and overhanging the left wing of the Carthaginians and the right of the Romans were some naked hills, which were not objects of suspicion to either party, as they had no wood upon them, nor any hiding-places for an ambuscade. [5] In the plain which lay between them skirmishes hardly worth mentioning took place between parties sallying from the outposts. It was evident that what the Roman aimed at was to prevent the enemy from going off, while Hannibal, who was desirous of escaping thence, came down with all his forces, and formed in order of battle. [6] Upon this the consul, imitating the crafty character of his enemy, ordered five cohorts, with the addition of five maniples, to pass the summit by night and sit down in the valleys on the opposite side; a measure to which he was prompted the more strongly in proportion as he felt that there could exist no suspicion of an ambuscade in hills so uncovered. [7] Of the time for rising up from their retreat and of falling upon the enemy he informed Tiberius Claudius Asellus, [p. 1148]a military tribune, and Publius Claudius, praefect of the allies, whom he sent with them. [8] The general himself, at break of day, drew out all his forces, both foot and horse, for battle. Shortly after, the signal for battle was given out by Hannibal, and a noise was raised in the camp, from the troops running hastily to arms; then both horse and foot eagerly rushed through the gates, and spreading themselves over the plain, hastened to the enemy. [9] The consul perceiving them thus disordered, gave orders to Caius Aurunculeius, a military tribune of the third legion, to send out the cavalry of the legion to charge the enemy with all possible vehemence, for that the enemy had spread themselves [10??] like cattle in such disorder throughout the whole plain, that they might be knocked down and trampled under foot before they could be formed.