Strabo Geography 7.3.5

[5] In fact, it is said that a certain man of the Getae, Zamolxis by name, had been a slave to Pythagoras, and had learned some things about the heavenly bodies from him, ¹ as also certain other things from the Egyptians, for in his wanderings he had gone even as far as Egypt; and when he came on back to his home-land he was eagerly courted by the rulers and the people of the tribe, because he could make predictions from the celestial signs; and at last he persuaded the king to take him as a partner in the government, on the ground that he was competent to report the will of the gods; and although at the outset he was only made a priest of the god who was most honored in their country, yet afterwards he was even addressed as god, and having taken possession of a certain cavernous place that was inaccessible to anyone else he spent his life there, only rarely meeting with any people outside except the king and his own attendants; and the king cooperated with him, because he saw that the people paid much more attention to himself than before, in the belief that the decrees which he promulgated were in accordance with the counsel of the gods. This custom persisted even down to our own time, because some man of that character was always to be found, who, though in fact only a counsellor to the king, was called god among the Getae. And the people took up the notion that the mountain ² was sacred and they so call it, but its name is Cogaeonum, ³ like that of the river which flows past it. So, too, at the time when Byrebistas, ⁴ against whom already ⁵ the Deified Caesar had prepared to make an expedition, was reigning over the Getae, the office in question was held by Decaeneus, and somehow or other the Pythagorean doctrine of abstention from eating any living thing still survived as taught by Zamolxis.