Ymddiddan Gwyddno Garanhir ac/a Gwyn fab/ap Nudd (The Dialogue of Gwyddneu/Gwyddno Garanhir and Gwyn ap Nudd), from Llyfr Du Cærfyrddin (Black Book of Cærmarthen). Tenth/Eleventh Century AD:

‘Ymddiddan Gwyddno Garanhir a Gwyn ap Nudd: Llyfr Du Caerfyrddin XXXIII’ at Mary Jones.
https://www.maryjones.us/ctexts/bbc33w.html

‘The Dialogue of Gwyddno Garanhir and Gwyn ap Nudd: The Black Book of Carmarthen XXXIII from The Four Ancient Books of Wales’ at Mary Jones.
https://www.maryjones.us/ctexts/bbc33.html