Caesar - Gallic War - Book I, 23 - Bibracte
On the morrow, as no more than two days remained before it was proper to issue the corn ration to the troops, and as he was no more than eighteen miles from Bibracte, by far the largest and the best-provided of the Aeduan towns, he considered that he must attend to the corn supply. He therefore turned his line of march away from the Helvetii, and made with all speed for Bibracte. The change was reported to the enemy by some deserters from Lucius Aemilius, a troop leader of the Gallic horse. Now the Helvetii may have supposed that the Romans were moving away from them because of sheer panic, the more so because on the day before they had not joined battle after seizing the higher ground; or they may have believed that the Romans could be cut off from their corn supply. Whichever the reason, they changed their plan, altered their route, and began to pursue and to annoy the Roman rearguard.