Book I, Chapter 22

And the Celtic Druids investigated to the very highest point the Pythagorean philosophy, after Zamolxis, by birth a Thracian, a servant of Pythagoras, became to them the originator of this discipline. Now after the death of Pythagoras, Zamolxis, repairing thither, became to them the originator of this philosophy. The Celts esteem these as prophets and seers, on account of their foretelling to them certain (events), from calculations and numbers by the Pythagorean art; on the methods of which very art also we shall not keep silence, since also from these some have presumed to introduce heresies; but the Druids resort to magical rites likewise.