This must be one of them. Clement of Alexandria,
Miscellanies 4.9:
Περὶ δὲ τοῦ μαρτυρίου διαρρήδην ὁ κύριος εἴρηκεν, καὶ τὰ διαφόρως γεγραμμένα συντάξωμεν· «λέγω δὲ ὑμῖν, πᾶς ὃς ἐὰν ὁμο- λογήσῃ ἐν ἐμοὶ ἔμπροσθεν τῶν ἀνθρώπων, καὶ ὁ υἱὸς τοῦ ἀνθρώπου ὁμολογήσει ἐν αὐτῷ ἔμπροσθεν τῶν ἀγγέλων τοῦ θεοῦ· τὸν δὲ ἀρνησάμενόν με ἐνώπιον τῶν ἀνθρώπων ἀπαρνήσομαι αὐτὸν ἔμπρο- σθεν τῶν ἀγγέλων.» «ὃς γὰρ ἂν ἐπαισχυνθῇ με ἢ τοὺς ἐμοὺς λόγους ἐν τῇ γενεᾷ ταύτῃ τῇ μοιχαλίδι καὶ ἁμαρτωλῷ, καὶ ὁ υἱὸς τοῦ ἀν- θρώπου ἐπαισχυνθήσεται αὐτόν, ὅταν ἔλθῃ ἐν τῇ δόξῃ τοῦ πατρὸς αὐτοῦ μετὰ τῶν ἀγγέλων αὐτοῦ.» «πᾶς οὖν ὅστις ἐὰν ὁμολογήσῃ ἐν ἐμοὶ ἔμπροσθεν τῶν ἀνθρώπων, ὁμολογήσω κἀγὼ ἐν αὐτῷ ἔμ- προσθεν τοῦ πατρός μου τοῦ ἐν οὐρανοῖς.» «ὅταν δὲ φέρωσιν ὑμᾶς εἰς τὰς συναγωγὰς καὶ τὰς ἀρχὰς καὶ τὰς ἐξουσίας, μὴ προμεριμνᾶτε πῶς ἀπολογηθῆτε ἢ τί εἴπητε· τὸ γὰρ ἅγιον πνεῦμα διδάξει ὑμᾶς ἐν αὐτῇ τῇ ὥρᾳ τί δεῖ εἰπεῖν.» Τοῦτον ἐξηγούμενος τὸν τόπον Ἡρακλέων ὁ τῆς Οὐαλεντίνου σχολῆς δοκιμώτατος κατὰ λέξιν φησὶν ὁμολογίαν εἶναι τὴν μὲν ἐν πίστει καὶ πολιτείᾳ, τὴν δὲ ἐν φωνῇ. «ἡ μὲν οὖν ἐν φωνῇ ὁμολογία καὶ ἐπὶ τῶν ἐξουσιῶν γίνεται, ἣν μόνην, φησίν, ὁμολογίαν ἡγοῦνται εἶναι οἱ πολλοὶ οὐχ ὑγιῶς, δύνανται δὲ ταύτην τὴν ὁμολογίαν καὶ οἱ ὑποκριταὶ ὁμολογεῖν. ἀλλ' οὐδ' εὑρεθήσεται οὗτος ὁ λόγος καθο- λικῶς εἰρημένος· οὐ γὰρ πάντες οἱ σῳζόμενοι ὡμολόγησαν τὴν διὰ τῆς φωνῆς ὁμολογίαν καὶ ἐξῆλθον, ἐξ ὧν Ματθαῖος, Φίλιππος, Θωμᾶς, Λευῒς καὶ ἄλλοι πολλοί. καὶ ἔστιν ἡ διὰ τῆς φωνῆς ὁμολογία οὐ καθολική, ἀλλὰ μερική. καθολικὴ δὲ ἣν νῦν λέγει, ἡ ἐν ἔργοις καὶ πράξεσι καταλλήλοις τῆς εἰς αὐτὸν πίστεως. ἕπεται δὲ ταύτῃ τῇ ὁμολογίᾳ καὶ ἡ μερικὴ ἡ ἐπὶ τῶν ἐξουσιῶν, ἐὰν δέῃ καὶ ὁ λόγος αἱρῇ. ὁμολογήσει γὰρ οὗτος καὶ τῇ φωνῇ, ὀρθῶς προομολογήσας πρότερον τῇ διαθέσει. καὶ καλῶς ἐπὶ μὲν τῶν ὁμολογούντων «ἐν ἐμοὶ» εἶπεν, ἐπὶ δὲ τῶν ἀρνουμένων τὸ «ἐμὲ» προσέθηκεν. οὗτοι γάρ, κἂν τῇ φωνῇ ὁμολογήσωσιν αὐτόν, ἀρνοῦνται αὐτόν, τῇ πράξει μὴ ὁμολογοῦντες. μόνοι δ' ἐν αὐτῷ ὁμολογοῦσιν οἱ ἐν τῇ κατ' αὐτὸν πολιτείᾳ καὶ πράξει βιοῦντες, ἐν οἷς καὶ αὐτὸς ὁμολογεῖ ἐνει- λημμένος αὐτοὺς καὶ ἐχόμενος ὑπὸ τούτων. διόπερ ἀρνήσασθαι αὐτὸν οὐδέποτε δύνανται· ἀρνοῦνται δὲ αὐτὸν οἱ μὴ ὄντες ἐν αὐτῷ. οὐ γὰρ εἶπεν «ὃς ἀρνήσηται ἐν ἐμοί», ἀλλ' «ἐμέ»· οὐδεὶς γάρ ποτε ὢν ἐν αὐτῷ ἀρνεῖται αὐτόν. τὸ δὲ «ἔμπροσθεν τῶν ἀνθρώπων,» καὶ τῶν σῳζομένων καὶ τῶν ἐθνικῶν δὲ ὁμοίως παρ' οἷς μὲν καὶ τῇ πολιτείᾳ, παρ' οἷς δὲ καὶ τῇ φωνῇ. [διόπερ ἀρνήσασθαι αὐτὸν οὐδέ- ποτε δύνανται· ἀρνοῦνται δὲ αὐτὸν οἱ μὴ ὄντες ἐν αὐτῷ.]» Ταῦτα μὲν ὁ Ἡρακλέων.
On martyrdom the Lord has spoken explicitly, and what is written in different places we bring together. But I say unto you, Whosoever shall confess in Me before men, the Son of man also shall confess before the angels of God; but whosoever shall deny Me before men, him will I deny before the angels. Luke 12:8 Whosoever shall be ashamed of Me or of My words in this adulterous and sinful generation, of him shall the Son of man also be ashamed when He comes in the glory of His Father with His angels. Whosoever therefore shall confess in Me before men, him will I also confess before my Father in heaven. Matthew 10:32 And when they bring you before synagogues, and rulers, and powers, think not beforehand how you shall make your defence, or what you shall say. For the Holy Spirit shall teach you in the same hour what you must say. Luke 12:11-12 In explanation of this passage, Heracleon, the most distinguished of the school of Valentinians, says expressly, that there is a confession by faith and conduct, and one with the voice. The confession that is made with the voice, and before the authorities, is what the most reckon the only confession. Not soundly: and hypocrites also can confess with this confession. But neither will this utterance be found to be spoken universally; for all the saved have confessed with the confession made by the voice, and departed. Of whom are Matthew, Philip, Thomas, Levi, and many others. And confession by the lip is not universal, but partial. But that which He specifies now is universal, that which is by deeds and actions corresponding to faith in Him. This confession is followed by that which is partial, that before the authorities, if necessary, and reason dictate. For he will confess rightly with his voice who has first confessed by his disposition. And he has well used, with regard to those who confess, the expression 'in Me,' and applied to those who deny the expression 'Me.' For those, though they confess Him with the voice, yet deny Him, not confessing Him in their conduct. But those alone confess 'in Him,' who live in the confession and conduct according to Him, in which He also confesses, who is contained in them and held by them. Wherefore 'He never can deny Himself.' And those deny Him who are not in Him. For He said not, 'Whosoever shall deny' in Me, but 'Me.' For no one who is in Him will ever deny Him. And the expression 'before men?' applies both to the saved and the heathen similarly by conduct before the one, and by voice before the other. Wherefore they never can deny Him. But those deny Him who are not in Him. So far Heracleon.