Chronica Boemorum
Kurt cites the relevant passage (p. 6):
‘The Gyon River ... surrounds the country of Ethiopia, where are only black men, and which is called the land of Prester John. It is thought that this river is the Nile, which descends to Egypt, across a place called Abasty [Abasia, Abyssinia], where are Christians of the apostle St Matthew.’
From Silverberg (p. 170):
John de' Marignolli wrote an account of his Eastern travels which, in the words of his nineteenth-century translator Henry Yule, is to be found, 'like unexpected fossils in a mud-bank, imbedded in a Chronicle of Bohemia' that he composed about 1355. This lengthy interpolation enters the chronicle on the flimsiest of pretenses: Marignolli chooses to begin his history of Bohemia with the Creation and the expulsion of Adam and Eve from Eden, and, having got as far as the statement that Eden is located "beyond India," is reminded that he himself once visited India, which leads him to say, "And now to insert some brief passages of what I have seen myself ... " An anecdotal description of the Orient that runs to more than fifty printed pages follows.