Einhard. The Life of the Emperor Charles. Chapter 10.
Charles also subdued the Bretons, who live on the sea coast, in the extreme western part of Gaul. When they reufsed to obey him, he sent an army against them, and compelled them to give hostages, and to promise to do his bidding. He afterwards entered Italy in person with his army, and passed through Rome to Capua, a city in Campania, where he pitched his camp and threatened the Beneventans with hostilities unless they should submit themselves to him. Their duke, Aragis, escaped the danger by sending his two sons, Rumold and Grimold, with a great sum of money to meet the King, begging him to accept them as hostages, and promising for himself and his people compliance with all the King's commands, on the single condition that his personal attendance should not be required. The King took the welfare of the people into account rather than the stubborn disposition of the Duke, accepted the proferred hostages, and released him from the obligation to appear before him in consideration ofh is handsome gift. He retained the younger son only as hostage, and sent the elder back to his father, and returned to Rome, leaving commissioners with Aragis to exact the oath of allegiance, and administer it to the Beneventans. He stayed in Rome several days in order to pay his devotions at the holy places, and then came back to Gaul.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.