Saxo Grammaticus and the men of Thule
Saxo Grammaticus (d. c. 1220) was a Danish historian working for Archbishop Absalon (d. 1201), the main advisor to King Valdemar I of Denmark (r. 1154–1182). Saxo is the author of the monumental history of Denmark known as the Gesta Danorum ('Deeds of the Danes') written in Latin, the lingua franca of the medieval scholarship.
The Gesta Danorum consists of sixteen books, the first nine of which deal with the legendary past of Denmark, from the legendary ruler Dan to Gorm the Old, the first historical king of Denmark to whom the current Royal House traces its genealogy (see the Royal Lineage on the official webpage of the Danish royal house).
The first, legendary part of the chronicle includes stories that have their counterparts in Old Norse-Icelandic literature, suggesting at least some common sources for Saxo's accounts and the sagas. Moreover, Saxo explicitly stated in the Gesta Danorum that he was familiar with Icelandic sources and extensively used them in his own work.
There are a few indications of Saxo's familiarity with the Icelandic tradition. In the introduction to the Gesta Danorum, where Saxo outlines his motivation and sources, he wrote:
Nor may the pains of the men of Thule be blotted in oblivion; [...] they account it a delight to learn and to consign to remembrance the history of all nations, deeming it as great a glory to set forth the excellences of others as to display their own. Their stores, which are stocked with attestations of historical events, I have examined somewhat closely, and have woven together no small portion of the present work by following their narrative.
Saxo, Gesta Danorum, Preface, translated by Oliver Elton.
The men of Thule, mentioned in this passage, are interpreted as the people of Iceland (Icelanders) and Saxo's account of their learned literature can be considered as evidence that he used some Icelandic sources in his chronicle.
Moreover, in book 14 Saxo mentions an associate of Absalon, an Icelander named Arnoldus, who was well-versed in telling the old stories. Arnoldus can be identified with Arnaldur Þorvaldsson, skáld (poet) at the court of King Valdemar the Great. Arnaldur is listed as Valdemar's skáld in Skáldatal, a catalogue of poets compiled in the thirteenth century. It may be that Saxo learned of his Icelandic sources through Arnaldur.