Einhard. The Life of the Emperor Charles. Chapter 12.
After these uprisings had been thus quelled, war was declared against the Slavs who are commonly known among us as Wilzi, but properly, that is to say in their own tongue, are called Welatabians. The Saxons served in this campaign as auxiliaries among the tribes that followed the King's standard at his summons, but their obedience lacked sincerity and devotion. War was declared because the Slavs kept harassing the Abodriti, old allies of the Franks, by continual raids, in spite of all commands to the contrar. A gulf of unknown length, but nowhere more than a hundred miles wide, and in many parts narrower, stretches off towards the east from the Western Ocean. Many tribes have settlements on its shores; the Danes and Swedes, whom we call Northemn, on the northern shore and all the adjacent islands; but the southern shore is inhabited by the Slavs and Aisti, and various other tribes. The Welatabians, against whom the King now made war, were the chief of these; but in a single campaign, which he conducted in person, he so crushed and subdued them that they did not think it advisable thereafter to refuse obedience to his commands.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.