Here is the source:

Charles [the Fat], who had the title of emperor, marched against the Northmen with a large army and advanced right up to their fortification. Once he had got there, however, his courage failed him. Through the intervention of certain men, he managed to reach an agreement with Godefrid and his men on the following terms: namely that Godefrid would be baptised, and would then receive Frisia and the other regions that Roric had held. To Sigfrid and Gorm and their accomplices he gave several thousand pounds of silver and gold which he had seized from the treasury of St-Stephen at Metz and from the resting-places of other saints, and he gave them permission to stay so that they could go on ravaging a part of his cousin’s kingdom as they had before.

I've googled it and I get references to the Vikings and pirates; I also seem to remember reading something similar about the Danes in Einhard, and Godefrid being their leader - but he was assasinated and I think it was before Charles the Fat?