Pliny the Elder, Natural History 7.19
haut procul urbe Roma in Faliscorum agro familiae sunt paucae quae vocantur Hirpi. hae sacrificio annuo, quod fit ad montem Soractem Apollini, super ambustam ligni struem ambulantes non aduruntur, et ob id perpetuo senatus consulto militiae omniumque aliorum munerum vacationem habent.
Not far from the city of Rome, in the territory of the Falisci, there are a few families who are called Hirpi. At the annual sacrifice performed by these families, which happens in honour of Apollo on Mount Soracte, they walk on a burning heap of wood but are not burnt. For this reason, by a decree of the senate in perpetuity, they are exempt from military service and all other duties.