Einhard. The Life of the Emperor Charles. Chapter 9.
In the midst of this vigorous and almost uninterrupted struggle with the Saxons, he covered the frontier by garrisons at the proper points, and marched over the Pyrenees into Spain at the head of all the forces tha he could muster. All the towns and castles that he attacked surrendered, and up to the time of his homeward march he sustained no loss whatever; but on his return through the Pyrenees he had cause to rue the treachery of the Gascons. That region is well adapted for ambuscades by reason of the thick forests that cover it; and as the army was advancing in the long line of march necessitated by the narrowness of the road, the Gascons, who lay in ambush on the top of a very high mountain, attacked the rear of the baggage train and the rear guard in charge of it, and hurled them down to the very bottom of the valley. In the struggle that ensued, they cut them off to a man; they then plundered the baggage, and dispersed with all speed in every direction under cover of approaching night. The lightness of their armor and the nature of the battle ground stood the Gascons in good stead on this occasion, whereas the Franks fought at a disadvantage in every respect, because of the weight of their armor and the unevenness of the ground. Eggihard, the King's steward; Anselm, Count Palatine; and Roland, Governor of the March of Brittany, with very many others, fell in this engagement. This ill turn could not be avenged for the nonce, because the enemy scattered so widely after carrying out their plan that not the least clue could be had to their whereabouts.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.