The Histories
Herodotus

Book 2: On the Ister (Danube)

XXXIII. This is enough of the story told by Etearchus the Ammonian; except he said that the Nasamonians returned, as the men of Cyrene told me, and that the people to whose country they came were all wizards; [2] as to the river that ran past the city, Etearchus guessed it to be the Nile; and reason proves as much. For the Nile flows from Libya, right through the middle of it; and as I guess, reasoning about things unknown from visible signs, it rises proportionally as far away as does the Ister. [3] For the Ister flows from the land of the Celts and the city of Pyrene through the very middle of Europe; now the Celts live beyond the Pillars of Heracles, being neighbors of the Cynesii, who are the westernmost of all the peoples inhabiting Europe. [4] The Ister, then, flows clean across Europe and ends its course in the Euxine sea, at Istria, which is inhabited by Milesian colonists.

XXXIV. The Ister, since it flows through inhabited country, is known from many reports; but no one can speak of the source of the Nile; for Libya, though which it runs, is uninhabited and desert. Regarding its course, I have related everything that I could learn by inquiry; and it issues into Egypt. Now Egypt lies about opposite to the mountainous part of Cilicia; [2] from there, it is a straight five days' journey for an unencumbered man to Sinope on the Euxine; and Sinope lies opposite the place where the Ister falls into the sea. Thus I suppose the course of the Nile in its passage through Libya to be like the course of the Ister.

XXXV. It is sufficient to say this much concerning the Nile. But concerning Egypt, I am going to speak at length, because it has the most wonders, and everywhere presents works beyond description; therefore, I shall say the more concerning Egypt.

Book 4: On the Ister (Danube)

XLIX. These are the native-born Scythian rivers that help to swell it; but the Maris river, which commingles with the Ister, flows from the Agathyrsi. The Atlas, Auras, and Tibisis, three other great rivers that pour into it, flow north from the heights of Haemus. The Athrys, the Noes, and the Artanes flow into the Ister from the country of the Crobyzi in Thrace; the Cius river, which cuts through the middle of Haemus, from the Paeonians and the mountain range of Rhodope. [2] The Angrus river flows north from Illyria into the Triballic plain and the Brongus river, and the Brongus into the Ister, which receives these two great rivers into itself. The Carpis and another river called Alpis also flow northward, from the country north of the Ombrici, to flow into it; [3] for the Ister traverses the whole of Europe, rising among the Celts, who are the most westerly dwellers in Europe, except for the Cynetes, and flowing thus clean across Europe it issues forth along the borders of Scythia.

SOURCE
Herodotus, with an English translation by A. D. Godley. Cambridge. Harvard University Press. 1920. From Perseus