While no formal announcement can be found, it is clear that the thinking of the apostles was so thoroughly Trinitarian that it fairly bubbled up in their wording, almost no matter what they wrote.
Πέτρος ἀπόστολος Ἰησοῦ Χριστοῦ ἐκλεκτοῖς παρεπιδήμοις διασπορᾶς Πόντου, Γαλατίας, Καππαδοκίας, Ἀσίας καὶ Βιθυνίας, 2 κατὰ πρόγνωσιν θεοῦ πατρὸς ἐν ἁγιασμῷ πνεύματος εἰς ὑπακοὴν καὶ ῥαντισμὸν αἵματος Ἰησοῦ Χριστοῦ, χάρις ὑμῖν καὶ εἰρήνη πληθυνθείη.There it is in verse two: Father, Spirit, Son. Once one begins to notice this, he discovers it unconsciously all throughout the apostles' writings, and many times in one breath, just as here (cf. Ephesians 2:18—ὅτι δι᾽ αὐτοῦ [sc. through Christ] ἔχομεν τὴν προσαγωγὴν οἱ ἀμφότεροι ἐν ἑνὶ πνεύματι πρὸς τὸν πατέρα).