Chronica Boemorum
Specifically, Marignolli associates the biblical River Gyon with the Nile, and describes the river as "that which circleth the land of Ethiopia" and "where are now the Negroes, and which is called the Land of Prester John... The Christians of St. Matthew the Apostle are there, and the Sultan pays them tribute on account of the river, because they have it in their power to shut off the water and then Egypt would perish" (qtd. in Silverberg, p. 170).
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.
