These quotes by St Cyprian of Carthage in c250 should do the trick:
“The Lord says to Peter: ‘I say to you,’ he says, ‘that you are Peter, and upon this rock I will build my Church, and the gates of hell will not overcome it. And to you I will give the keys of the kingdom of heaven; and whatever things you bind on earth shall be bound also in heaven, and whatever you loose on earth, they shall be loosed also in heaven’ [Matt. 16:18–19]). … On him [Peter] he builds the Church, and to him he gives the command to feed the sheep [John 21:17], and although he assigns a like power to all the apostles, yet he founded a single chair [cathedra], and he established by his own authority a source and an intrinsic reason for that unity. Indeed, the others were also what Peter was *, but a primacy is given to Peter, whereby it is made clear that there is but one Church and one chair. So too, all [the apostles] are shepherds, and the flock is shown to be one, fed by all the apostles in single-minded accord. If someone does not hold fast to this unity of Peter, can he imagine that he still holds the faith? If he [should] desert the chair of Peter upon whom the Church was built, can he still be confident that he is in the Church?” (The Unity of the Catholic Church 4; 1st edition [A.D. 251]).
“Cyprian to [Pope] Cornelius, his brother. Greeting. . . . We decided to send and are sending a letter to you from all throughout the province [where I am] so that all our colleagues might give their decided approval and support to you and to your communion, that is, to both the unity and the charity of the Catholic Church” (Letters 48:1, 3 [A.D. 253]).
“Cyprian to Antonian, his brother. Greeting … You wrote … that I should forward a copy of the same letter to our colleague [Pope] Cornelius, so that, laying aside all anxiety, he might at once know that you held communion with him, that is, with the Catholic Church” (ibid., 55:1).
“Cornelius was made bishop by the decision of God and of his Christ, by the testimony of almost all the clergy, by the applause of the people then present, by the college of venerable priests and good men … when the place of Fabian, which is the place of Peter, the dignity of the sacerdotal chair, was vacant. Since it has been occupied both at the will of God and with the ratified consent of all of us, whoever now wishes to become bishop must do so outside [the Church]. For he cannot have ecclesiastical rank who does not hold to the unity of the Church” (ibid., 55:8).
“With a false bishop appointed for themselves by heretics, they dare even to set sail and carry letters from schismatics and blasphemers to the chair of Peter and to the principal church [at Rome], in which sacerdotal unity has its source” (ibid., 59:14).
Though if you get asked for something earlier there’s this by St Ireaneus of Lyon:
“But since it would be too long to enumerate in such a volume as this the succession of all the churches, we shall confound all those who, in whatever manner, whether through self-satisfaction or vainglory, or through blindness and wicked opinion, assemble other than where it is proper, by pointing out here the successions of the bishops of the greatest and most ancient church known to all, founded and organized at Rome by the two most glorious apostles, Peter and Paul, that church which has the tradition and the faith which comes down to us after having been announced to men by the apostles. With that church, because of its superior origin, all the churches must agree, that is, all the faithful in the whole world, and it is in her that the faithful everywhere have maintained the apostolic tradition” (Against Heresies 3:3:2 [A.D. 189]).
There are earlier quotes but they are less obvious, but these are all before Constantine. Hope that helps.*