Eusebius On a Collection of Letters of Origen He Personally Catalogued in the Library of Caesarea

6.36.3 φέρεται δὲ αὐτοῦ καὶ πρὸς αὐτὸν βασιλέα Φίλιππον ἐπιστολὴ καὶ ἄλλη πρὸς τὴν τούτου γαμετὴν Σευήραν διάφοροί τε ἄλλαι πρὸς διαφόρους· ὧν ὁπόσας σποράδην παρὰ διαφόροις σωθείσας συναγαγεῖν δεδυνήμεθα, ἐν ἰδίαις τόμων περιγραφαῖς, ὡς ἂν μηκέτι διαρρίπτοιντο, κατελέξαμεν, τὸν ἑκατὸν ἀριθμὸν ὑπερβαινούσας. 6.36.4 γράφει δὲ καὶ Φαβιανῷ τῷ κατὰ Ῥώμην ἐπισκόπῳ ἑτέροις τε πλείστοις ἄρχουσιν ἐκκλησιῶν περὶ τῆς κατ' αὐτὸν ὀρθοδοξίας. ἔχεις καὶ τούτων τὰς ἀποδείξεις ἐν ἕκτῳ τῆς γραφείσης ἡμῖν περὶ τοῦ ἀνδρὸς ἀπολογίας. 

It is also reported that he wrote a letter to Emperor Philip and another to his wife Severa, as well as various other letters to different individuals. We have managed to collect as many of these scattered letters as possible from different sources, cataloging them in separate volumes to prevent their loss, exceeding one hundred in number. He also wrote to Fabian, the bishop of Rome, and to many other church leaders about his orthodoxy. You can find these proofs in the sixth book of our written defense of the man.