Titus Livius (Livy), The History of Rome, Book 35
Cyrus Evans, Ed.

4. While the operations remained at a stand at Pisae, the other consul, Lucius Cornelius Merula, led his army through the extreme borders of the Ligurians, into the territory of the Boians, where the mode of proceeding was quite the reverse of that which took place in the war of Liguria. [2] The consul took the field; the enemy refused to fight; and the Romans, when no one would come out against them, went out in parties to plunder, while the Boians chose to let their country be laid waste with impunity rather than venture an engagement in defence of it. [3] When all places were completely ravaged with fire and sword, the consul quitted the enemy's lands, and marched towards Mutina, in a careless manner, as through a pacific population. [4] The Boians, when they learned that the enemy had withdrawn beyond their frontiers, followed him as secretly as possible, watching an opportunity for an ambuscade; and, having gone by his camp in the night, took possession of a defile through which the Romans were to pass. [5] But as they were not able to effect this with sufficient secrecy, the consul, who usually began his march late in the night, now waited until day, lest, in the disorderly fight likely to ensue, darkness might increase the confusion; and though he did not stir before it was light, yet he sent forward a troop of horse to explore the country. [6] When intelligence was brought by them of the number and situation of the enemy, he ordered the baggage to be heaped together in the centre, and the veterans to throw up a rampart round it; and then, with the rest of the army in order of battle, he advanced towards the enemy. [7] The Gauls did the same, when they found that their stratagem was detected, and that they were to engage in a fair and regular battle, where success must depend on valour alone.