Each of the complete manuscripts of Cyfraith Hywel includes a prologue. The prologue describes how Hywel Dda gathered together the most learned men in Wales in order to organise the laws; it is highly unlikely that this actually happened, and the purpose of the prologues was to emphasize a royal and Christian origin and background to the laws, and that in the face of criticism of the laws from outside Wales. The story in the prologues increases with time, with more details in the later versions. The prologues are the main piece of evidence for the link between Hywel Dda and the laws.
Pryce, H., ‘The Prologues to the Welsh Lawbooks’, Bulletin of the Board of Celtic Studies 33 (1986), 151-182.
Russell, P., The Prologues to the Mediaeval Welsh Lawbooks, Basic Texts for Brittonic History 3 (Cambridge, Department of Anglo-Saxon, Norse, and Celtic, 2004).