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

33. Having transported the wounded to the town of Aebura, he led the legions through Carpetania toward Contrebia. [2] When this town, on being besieged, had asked aid from the Celtiberians and they were slow in coming, not because they dawdled, but because after they had left home they were held up by roads made impassable by continuous rains and by swollen rivers, despairing of assistance from its friends, it surrendered. [3] Flaccus, too, having been driven to do so by dire storms, led his whole army into the city. [4] The Celtiberians, who had set out from home in ignorance of the surrender, when they had at last arrived at Contrebia after crossing the rivers as soon as the rains ceased, seeing no camp outside the walls and concluding that the enemy had either gone around to the other side or retired, carelessly approached the town in scattered bodies. [5] The Romans made a sally against them from two gates and attacking them while they were in disorder put them to flight. [6] The circumstance which hampered their resisting and standing up to the fight, namely, that they were not advancing in one column or with large numbers grouped in formation around the standards, was itself the cause of safety for the majority in the flight: [7] for scattered as they were anyhow over the whole plain, they broke up into groups [p. 103]and fled, and nowhere did the enemy surround a¹ compact mass. Nevertheless about twelve thousand were killed, more than five thousand men captured, with four hundred horses and sixty-two military standards. [8] Those who came straggling home after their rout, by describing the surrender of Contrebia and their own disaster, prevented the departure of another force of Celtiberians who were coming. [9] All immediately dispersed to their villages and strongholds. Flaccus, setting out from Contrebia on a raid, led the legions through Celtiberia, capturing many forts, until the greatest part of the Celtiberians had surrendered.