This is found in Geoffrey of Monmouth's Historia Regum Brittaniae (History of the Kings of Britain. See Other Works). Geoffrey uses Arthur's battle as an excuse to bring in three watery Wonders from Nennius (Lake Lumonoy, Fount Guur Helic, and Llyn Liuan) though it is unclear where he gets the information about the prophetic gathering of the Eagles, so the version used may not be one we have (or may be in an unpublished manuscript). You can read the relevant sections of the Historia Regum Brittaniae here. The fact that there are three of these may suggest a traditional Welsh triad (see, for other examples, Bromwich*) on "the three marvellous lakes", though this is not apparent in Nennius.