Einhard. The Life of the Emperor Charles. Chapter 30.
Toward the close his life, when he was broken by ill-health and old age, he summoned Louis, King of Aquitania, his ownly surviving son by Hildegard, and gathered together all the chief men of the whole kingdom of the Franks in a solemn assembly. He appointed Louis , with their unanimous consent, to rule with himself over the whole kingdom, and constituted him heir to the imperial name; then, placing the diadem upon his son's head, he bade him be proclaimed Emperor and Augustus. This step was hailed by all present with great favor, for it really seemed as if God had prompted him to it for the kingdom's good; it increased the King's dignity, and struck no little terror into foreign nations. After sending his son back to Aquitania, although weak from age he set out to hunt, as usual, near his palace at Aix-la-Chapelle, and passed the rest of the autumn in the chase, returning thither about the first of November. While wintering there, he was seized, in the month of January, with a high fever, and took to his bed. As soon as he was taken sick, he prescribed for himself abstinence from food, as he always used to do in case of fever, thinking that the disease could be driven off, or at least mitigated, by fasting. Besides the fever, he suffered from a pain in the side, which the Greeks call pleurisy; but he still persisted in fasting, and in keeping up his strength only by draughts taken at very long intervals. He died January twenty-eighth, the seventh day from the time that he took to his bed, at nine o'clock in the morning, after partaking of the holy communion, in the seventy-second year of his age and the forty-seventh of his reign.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.