In 1 Clement 63:2, the author seems to teach that his epistle was written by divine inspiration:
For you will make us joyful and
happy if you become obedient to what we have written through the Holy Spirit (ἐὰν
ὑπήκοοι γενόμενοι τοῖς ὑφ᾽ ἡμῶν γεγραμμένοις διὰ τοῦ ἁγίου πνεύματος) . . .