Gildas Bandonicus, a Celtic monk, lived in the 6th century from about 494 AD to 570 AD.
Gildas is the only substantial source which survives through to today from the period of the Anglo-Saxon conquest of Britain, and the best source of information on the Dark Ages in Britain, prior to the books by Bede (who wrote his "Ecclesiastical History of the English People" almost 200 years later, in 731 AD)
The earliest account of the battle of Badon comes from Gildas, who wrote in his De excidio et conquestu Britanniae, about the battle in Mons Badonicus.
Gildas did not mention Arthur, the monk implied that the victor was Ambrosius Aurelianus.