Per the lack of evidence of Roman Petrine authority in the Ancient Church, I'd cite St. Irenaues of Lyons (although there are others, he is the most clear).
From, "Against the Heresies III, Preface - 4.2
“To enumerate the successions of all the churches woudl take up too much space in a volume of this kind. But in order to put to shame all those who in any way, either through self-conceit, or through vainglory, or through blind and evil opinion, gather as they should not, I need only cite of that very great, most ancient and universally known church founded and established at Rome by those two most glorious apostles Peter and Paul and draw attention to the tradition which that church has received from the apostles and to the faith it preaches which has come down to our time through the succession of bishops. For in view of the outstanding pre-eminence of this church, there cannot be any disagreement between it and every other church (that is, the faithful in every place) – every church, that is, in which men in every place have at all times preserved the apostolic tradition.”
The source of Irenaeus' instruction is also interesting:
http://www.religionfacts.com/christianity/people/irenaeus.htm
St. Irenaeus was bishop of Lugdunum in Gaul, which is now Lyons, France. He was a disciple of Polycarp, who himself was a disciple of John the Evangelist. He is recognized as a saint by both the Eastern Orthodox Church and the Catholic Church, and his writings were formative in the early development of Christian theology. His most famous work is Against Heresies, a lengthy description and refutation of Gnosticism.
-snip-
In his letter to Florinus (Eusebius, Hist. eccl., 5.20.5), Irenaeus speaks of having seen Polycarp at Smyrna in the emperor's train when he himself was still but a boy. The date of the death of Polycarp is now practically settled for 155. For various reasons, this emperor must have been Hadrian, who visited Asia Minor in 123 and 129, in the latter of which years the meeting must have taken place. All that Irenaeus tells of his recollections of Polycarp at this period shows that he must have been at least 12 or 15, and thus was probably born about 115. He implies distinctly that his intercourse with and instruction by Polycarp lasted for a number of years, very likely from about 129 to 150; and the same conclusion follows from what he tells of the teaching received in Asia Minor from certain disciples of the apostles.
There are two further passages (4.27.1-32 and 5.33.3-4) which can be understood only as asserting that he had this oral instruction from more than one of such disciples and when he was of an age to take it in and be deeply impressed by it.
Well, thank you! That was exactly the letter I was thinking of ("those two most glorious apostles Peter and Paul") but it would have taken me hours to find it!