Nennius/Nemnuuis/Ninnius/Nemnius (pseudo). Historia Brittonum (History of Britons). late Eighth/early Ninth Century AD (AD 828/829/830):

Giles, John Allen, tr. Nennius: History of the Britons (Historia Brittonum). Cambridge, Ontario: In Parentheses Publications (Medieval Latin Series), 2000.
http://www.yorku.ca/inpar/nennius_giles.pdf