Firmilian of Caesarea, Letter to Cyprian (Ep. 74/75) 6
Those at Rome do not observe in all things what has been handed down from the beginning, and they invoke the authority of the Apostles in vain — something anyone can recognize from the fact that, concerning the celebration of the Paschal days and many other divine sacraments, there are diversities among them, and not everything is observed at Rome exactly as it is at Jerusalem. Indeed, in many other provinces countless practices vary because of differences of place and name, yet this has never caused a breach of the peace and unity of the Catholic Church. What Stephen has now dared to do — breaking the peace with you, which his predecessors always preserved with you in mutual love and honor — and even slandering the blessed apostles Peter and Paul, as if they themselves had handed down this practice, when in their letters they cursed heretics and warned us to avoid them — from this it is clear that this tradition is merely human, a tradition that champions heretics and defends their possession of baptism, which belongs to the Church alone.
Latin
Eos autem qui Romae sunt non ea in omnibus observare quae sint ab origine tradita; et frustra Apostolorum auctoritatem praetendere scire quis etiam inde potest, quod circa celebrandos dies Paschae et circa multa alia divinae rei sacramenta videat esse apud illos aliquas diversitates, nec observari illic omnia aequaliter quae Hierosolymis observantur, secundum quod in caeteris quoque plurimis provinciis multa pro locorum et nominum diversitate variantur, nec tamen propter hoc ab Ecclesiae catholicae pace atque unitate aliquando discessum est. Quod nunc Stephanus ausus est facere, rumpens adversum vos pacem, quam semper antecessores ejus vobiscum amore et honore mutuo custodierunt, adhuc etiam infamans Petrum et Paulum beatos apostolos, quasi hoc ipsi tradiderint, qui in epistolis suis haereticos exsecrati sunt et ut eos evitemus monuerunt. Unde apparet traditionem hanc humanam esse quae haereticos asserit et baptisma, quod non nisi solius Ecclesiae est, eos habere defendit.