The Epistle (XL) of St Gregory I, Pope of Rome to Pope Eulogius of Alexandria.
Note well how Pope Gregory of Rome says that the Pope of Alexandria and the Patriarch of Antioch also have the Keys of Peter and all three are equal.
It's an interesting view of the Petrine office by a Pope of Rome who saw it as the common possession of all Three Patriarchates of ancient Christianity.
We sometime forget that before there were the Five Patriarchates there were originally just three and all were founded by Saint Peter: Rome, Alexandria and Antioch.
"Your most sweet Holiness [Pope Eulogius of Alexandria] has spoken much in your letter to me about the chair of Saint Peter, Prince of the apostles, saying that he himself now sits on it in the persons of his successors.
"And indeed I acknowledge myself to be unworthy, not only in the dignity of such as preside, but even in the number of such as stand. But I gladly accepted all that has been said, in that he has spoken to me about Peter's chair who occupies Peter's chair. And to him it is said by the voice of the Truth, To thee I will give the keys of the kingdom of heaven (Matth. xvi. 19). And again it is said to him, And when thou art converted, strengthen thy brethren (xxii. 32). And once more, Simon, son of Jonas, lovest thou Me? Feed my sheep (Joh. xxi. 17).
"Wherefore though there are many apostles, yet with regard to the principality itself the See of the Prince of the apostles alone has grown strong in authority, which in three places is the See of one. For he himself exalted the See in which he deigned even to rest and end the present life [Rome]. He himself adorned the See to which he sent his disciple as evangelist [Alexandria]. He himself established the See in which, though he was to leave it, he sat for seven years [Antioch]. Since then it is the See of one, and one See, over which by Divine authority three bishops now preside, whatever good I hear of you, this I impute to myself.” (Book VII, Epistle XL)