Einhard. The Life of the Emperor Charles. Chapter 3.
Pepin, however, was raised, by decree of the Roman Pontiff, from the rank of Mayor of the Palace to that of King, and ruled alone over the Franks for fifteen years or more. He died of dropsy, in Paris, at the close of the Aquitanian war, which he had waged with William, Duke of Aquitania, for nine successive years, and left two sons, Charles and Carloman, upon whom, by the grace of God, the succession devolved.
The Franks, in a general assembly of the people, made them both kings, on condition that they should divide the whole kingdom equally between them, Charles to take and rule the part that had belonged to their father, Pepin, and Carloman the part which their uncle, Carloman, had governed. The conditions were accepted, and each entered into possession of the share of the kingdom that fell to him by this arrangement; but peace was only maintained between them with the greatest difficulty, because many of Carloman's party kept trying to disturb their good understanding, and there were some even who plotted to involve them in a war with each other. The event, however, showed the danger to have been rather imaginary than real, for at Carloman's death his widow fled to Italy with her sons and her principal adherents, and without reason, despite her husband's brother, put herself and her children under the protection of Desiderius, King of the Lombards. Caroman had succumbed to disease after ruling two years in common with his brother, and at his death Charles was unanimously elected King of the Franks.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.