Einhard. The Life of the Emperor Charles. Chapter 5.
His first undertaking in a military way was the Aquitanian war, begun by his father, but not brought to a close; and because he thought that it could be readily carried through, he took it up while his brother was yet alive, calling upon him to render aid. The campaign once opened, he conducted it with the greatest vigor, notwithstanding his brother withheld the assistance that he had promised, and did not desist or shrink from his self-imposed task until, by his patience and firmness, he had completely gained his ends. He compelled Hunold, who had attempted to seize Aquitania after Waifar's death, and renew the war then almost concluded, to abandon Aquitania and flee to Gascony. Even here he gave him no rest, but crossed the River Garonne, built the castle of Fronsac, and sent ambassadors to Lupus, Duke of Gascony, to demand the surrender of the fugitive, threatening to take him by force unless he was promptly given up on him. Thereupon Lupus chose the wise course, and not only gave Hunold up, but submitted himself, with the province which he ruled, to the King.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.