Titus Livius (Livy), The History of Rome, Book 5
D. Spillan, A.M., M.D., Ed.

34. Concerning the passage of the Gauls into Italy we have heard as follows. In the reign of Tarquinius Priscus at Rome, the supreme government of the Celts, who compose the third part of Gaul, was in the hands of the Biturigians: they gave a king to the Celtic nation. [2] This was Ambigatus, one very much distinguished by his merit, and both his great prosperity in his own concerns and in those of the public; for under his administration Gaul was so fruitful and so well peopled, that so very great a population appeared scarcely capable of being restrained by any government. [3] He being now advanced in years, and anxious to relieve his kingdom of so oppressive a crowd, declares his intention to bend his sister's sons, Bellovesus and Sigovesus, two enterprising youths, into whatever settlements the gods should grant them by augury: [4] that they should take out with them as great a number of men as they pleased, so that no nation might be able to obstruct them in their progress. Then to Sigovesus the Hercynian forest was assigned by the oracle: to Bellovesus the gods marked out a much more cheering route into Italy. [5] He carried out with him from the Biturigians, the Arvernians, the Senonians, the Aeduans, the Ambarrians, the Carnutians, and the Aulercians, all that was superfluous in their population. Having set out with an immense force of horse and foot, he arrived in the country of the Tricastinians. [6] [p. 366]Next the Alps were opposed [to their progress], and I am not surprised that they should seem impassable, as they had never been climbed over through any path as yet, as far at least as tradition can extend, unless we are disposed to believe the stories regarding Hercules. [7] When the height of the mountains kept the Gauls there penned up as it were, and they were looking around [to discover] by what path they might pass into another world between the summits, which joined the sky, a religious scruple detained them, it having been announced to them that strangers in search of lands were attacked by the nation of the Salyans. [8] These were the Massilians, who had come by sea from Phocaea. The Gauls considering this an omen of their own fortune, assisted them in fortifying the ground which they had taken possession of on their first landing, covered with spacious woods. [9] They themselves crossed the Alps through the Taurinian and pathless forests; and having defeated the Etrurians not far from the Ticinus, on hearing that the land in which they had posted themselves was called Insubria, the same name as the Insubres, a canton of the Aedui: embracing the omen of the place, they built a city there, and called it Mediolanum.