References to Arthur before 1100 CE.
On the Ruin and Conquest of Britain — De Excidio et Conquestu Britanniae (Latin, c510-530 CE). A sermon by the British cleric St. Gildas. It makes reference to Ambrosius Aurelianus, who in later works is the brother of Uther Pendragon, and uncle of Arthur. It also refers to the Battle of Mons Badonicus (Mt. Baden, or Bath-hill), the battle where Arthur was the war leader who lead the Britons to victory.
De Excidio et Conquestu Britanniae — Wikipedia
Gildas de excidio et conquestu britanniae — text in Latin, Keith’s history pages
On the Ruin of Britain — Project Gutenberg.
Gildas' "On the Ruin of Britain" — PDF from Camelot On-line.
Elegy for Cynddylan — Marwnad Cynddylan (Middle Welsh, c650-850 CE). Elegy to the 7th century Prince of Powys. The poem includes a very early reference to Arthur.
The line mentioning Arthur is:
I used to have brothers. It was better when they were
the young whelps of great Arthur, the mighty fortress.
Cynddylan — Wikipedia
Marwnad Cynddylan — Text in Welsh and English, Keith’s history pages
The Gododdin — Y Gododdin (Welsh, 7th–11th century CE). Ascribed to the bard Aneirin. Appears in the Book of Aneirin. Some source give what is perhaps the first mention of Arthur in literature.
The line mentioning Arthur is:
He fed black ravens on the rampart of a fortress
Though he was no Arthur.
Among the powerful ones in battle
In the front rank, Gwawrddur was a palisade.
Y Gododdin — Wikipedia
Y Gododdin — Old and Middle Welsh and Williams' translation, Project Gutenberg
Y Gododdin — Skene's translation, Celtic Literature Collective
Y Gododdin — Clancy's translation, Celtic Literature Collective
The Spoils of Annwfn — Preiddeu Annwfn (Middle Welsh, c900 CE). Attributed to Taliesin. A poem telling the tale of an expedition by Arthur and his men to Annwfn (or Annwn), likely the Otherworld, in order to steal a magical cauldron. The story may be related to later Grail stories and the Abduction of Guinevere stories.
Preiddeu Annwfn — Wikipedia
Preiddeu Annwn: The Spoils of Annwn — Sarah Higley translation, the Camelot Project. Includes both Middle Welsh and English, as well as sound files.
Preiddeu Annwn — Welsh text, Celtic Literature Collective
The Raid on the Otherworld — English trans., Celtic Literature Collective
Who is the gatekeeper? — Pa Gur yv y Porthaur (Middle Welsh, 10th century CE). This poem relates exploits of Arthur's warrior Kay. Though this is recorded in the Black Book of Carmarthen, the poem has been dated to a much earlier source.
Sir Kay, section on Pa Gur yv y Porthaur — Wikipedia
Pa Gur — Middle Welsh, Celtic Literature Collective
Arthur and the Porter — Skene's translation, Celtic Literature Collective
Culhwch and Olwen — Culhwch ac Olwen (Middle Welsh, c1080-1100 CE). Anonymous. Present in both the White Book of Rhydderch (c1325 CE) and the Red Book of Hergest (c1400 CE). This is one of the only Celtic tales that pre-dates the French romances.
Early Arthurian literature written in the form of British history.
The History of the Britons — Historia Brittonum (Latin, c828 CE). Ascribed to Nennius. The Wonders of Britain is sometimes attached to the Historia. It is a brief list of geographical wonders, including two with connections to Arthur.
Historia Brittonum — Wikipedia
Historia Brittonum — text in Latin, Keith’s history pages
Historia Brittonum — Gunn translation.
History of the Britons — Giles translation, Project Gutenberg.
Nennius' "Historia Brittonum" — PDF from Camelot On-line.
Nennius' "The Wonders of Britain" — Andy Evans' Wonders of Britain site.
The Annals of Wales — Annales Cambriae (Latin, c970 CE). Anonymous. A chronical of events in Wales and in the rest of the British isles and Ireland. Three entries relate to Arthurian legend, referring to the battles of Badon (516 CE), Camlann (537 CE), and Armterid (573 CE).
Annales Cambriae — Wikipedia
Annales Cambriae — Latin text, John Williams ed.
Annales Cambriae — translation, Celtic Literature Collective