Titus Livius (Livy), The History of Rome, Book 40
Evan T. Sage, Ph.D. and Alfred C. Schlesinger, Ph.D., Ed.

40. There was a fierce fight on every side, but the results varied. The legions fought nobly and not less effectively the two squadrons;¹ the auxiliaries from the province were hard pressed by men armed in the same way but consisting of a much better class of soldiers, and could not hold their ground. [2] When the Celtiberians saw that with battle-lines arrayed and in pitched battle they [3??] were not a match for the legions, they made a thrust with the wedge, in which manner of fighting they are so powerful that against whatever troops they drive their charge they cannot be checked. Then even the legions were thrown into disorder and the line was almost broken. [4] When [p. 125]Flaccus saw this disorganization, he rode on his horse² to the legionary cavalry and shouted, “Unless there is some hope of aid in you, now all will be over with this army.” [5] When from all sides a shout was raised asking him why he did not say what he wanted done, and they would execute his orders without delay, “double the squadrons,”³ he said, “you cavalry of the two legions, and turn your horses loose against the wedge of the enemy, with which they are forcing back our men. [6] You will do this with the greater weight if you will give your horses loose rein against them; and this the records say that Roman cavalry have often done and won great glory for themselves.”⁴ [7] They obeyed his order and, dropping their reins, twice, from this side and from that, drove through, causing great slaughter to the enemy and breaking every one of their spears. [8] The wedge having been scattered, in which all their hope had rested, the Celtiberians were in panic, and almost giving up the fight were looking for a place for flight. [9] And when the cavalry of the allies saw that the Roman cavalry had performed so memorable an exploit, they too, inspired by the valour of the others, without waiting for an order let their horses loose upon the broken enemy. [10] Then indeed all the Celtiberians were turned to flight, and the Roman commander, seeing the backs of the enemy, vowed a temple to Fortuna Equestris⁵ and games to Jupiter Optimus Maximus. [11] The Celtiberians were slaughtered as they scattered in flight through the whole defile. Seventeen thousand of the enemy are reported to have been killed that day, more than [p. 127]three thousand seven hundred taken alive, and there⁶ were captured seventy-seven military standards and about six hundred horses. [12] The victorious army remained that day in its own camp. The victory was not without loss of soldiers: four hundred and seventy-two Roman soldiers fell, of the allies of the Latin confederacy one thousand and nineteen, and in addition three thousand of the auxiliary troops. [13] So the victorious army, having won once more its former glory, was conducted to Tarraco. [14] Tiberius Sempronius the praetor, who had arrived two days earlier, went out to meet Fulvius on his arrival and congratulated him on his conspicuous success in defence of the state. In perfect harmony they arranged which soldiers they should discharge and which they should hold in service. [15] Then Fulvius embarked the discharged troops on ships and set out for Rome and Sempronius led the legions into Celtiberia.