Gwarchan Cynfelyn (The Lay/Incantation of Cynfelyn/Cynvelyn), in Llyfr Aneirin/Aneurin (Book of Aneirin/Aneurin). Sixth/Seventh/Ninth/Tenth Centuries AD:

‘Gwarchan Cynfelyn (in Skene, William F, ed. The Four Ancient Books of Wales. Edinburgh: Edmonston and Douglas, 1868)’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/a04w.html

‘The Gwarchan of Cynfelyn from Book of Aneurin, IV (in Skene, William F, ed. The Four Ancient Books of Wales. Edinburgh: Edmonston and Douglas, 1868)’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/a04.html