Breuddwyd Macsen/Maxen (Wledig) (Dream of Macsen/Maxen (Wledig)), generally included in the Mabinogi (Mabinogion). Early Twelfth Century AD and earlier:

‘NLW MS. Peniarth 4 (The White Book of Rhydderch, part 2) – page 45r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Pen4&page=45r

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 172r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=172r

‘The Dream of Macsen (Translated by Lady Charlotte Guest)’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/macsen.html