Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[85] THE people mourned for Aaron thirty days, and when this mourning was over, Moses removed the army from that place, and came to the river Arnon, which, issuing out of the mountains of Arabia, and running through all that wilderness, falls into the lake Asphaltitis, and becomes the limit between the land of the Moabites and the land of the Amorites. This land is fruitful, and sufficient to maintain a great number of men, with the good things it produces. Moses therefore sent messengers to Sihon, the king of this country, desiring that he would grant his army a passage, upon what security he should please to require; he promised that he should be no way injured, neither as to that country which Sihon governed, nor as to its inhabitants; and that he would buy his provisions at such a price as should be to their advantage, even though he should desire to sell them their very water. But Sihon refused his offer, and put his army into battle array, and was preparing every thing in order to hinder their passing over Arnon.