Titus Livius (Livy), The History of Rome, Book 35
Evan T. Sage, PhD professor of latin and head of the department of classics in the University of Pittsburgh, Ed.

4. While the Ligurian war was at a standstill around Pisa, the other consul, Lucius Cornelius Merula, led his army through the farthest lands of the Ligures into the country of the Boii, where the war was conducted in a fashion far different from that in the war with the Ligures. [2] The consul marched out to offer battle, the enemy declined to engage; the Romans scattered to plunder when no one confronted them, and the Boii preferred the [p. 11]devastation of their lands without interference on their¹ part to the risk of a decisive battle while protecting them. [3] When everything had been sufficiently wasted with sword and fire, the consul retired from the enemy's country and marched toward Mutina, his column taking no precautions as if it was traversing a pacified country. [4] When the Boii saw that the enemy had withdrawn from their territory, they followed stealthily, seeking a place for an ambush. At night they passed the Roman camp and seized a defile through which the Romans had to march. [5] Since their precautions for secrecy were insufficient, the consul, who had been accustomed to break camp late in the night, waited for daylight, that darkness might not increase the terror from a sudden attack, and, although he was moving by day, he nevertheless sent out a troop of cavalry to reconnoitre. [6] When word came back how strong the enemy was and where he was stationed, he ordered the baggage of the whole column brought into the midst and the triarii² to construct a rampart around it, and with the remainder of the army in battle-array marched towards the enemy. [7] The Gauls did the same when they realized that their stratagem was discovered and that they would have to fight in regular and fair combat where constant courage would conquer.