Thank you. Any documents or citations with Pope St Gregory saying as much?
EPISTLE XL. To EULOGIUS, BISHOP. Gregory to Eulogius, Bishop of Alexandria. Your most sweet Holiness 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, though special honour to myself in no wise delights me, yet I greatly rejoiced because you, most holy ones, have given to yourselves what you have bestowed upon me. For who can be ignorant that holy Church has been made firm in the solidity of the Prince of the apostles, who derived his name from the firmness of his mind, so as to be called Petrus from petra. 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 [Mark] as evangelist [Alexandria]. He himself stablished 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. If you believe anything good of me, impute this to your merits, since we are one in Him Who says, That they all may be one, as Thou, Father, art in me, and I in thee that they also may be one in us (Joh. xvii. 21).
Moreover, in paying you the debt of salutation which is due to you, I declare to you that I exult with great joy from knowing that you labour assiduously against the barkings of heretics; and I implore Almighty God that He would aid your Blessedness with His protection, so as through your tongue. to uproot every root of bitterness from the bosom of holy Church, lest it should germinate again to the hindrance of many, and through it many should be defiled. For having received your talent you think on the injunction, Trade till I come (Luke xix. 13). I therefore, though unable to trade at all nevertheless rejoice with you in the gains of your trade, inasmuch as I know this, that if operation does not make me partaker, yet charity does make me a partaker in your labour. For I reckon that the good of a neighbour is common to one that stands idle, if he knows how to rejoice in common in the doings of the other. Furthermore, I have wished to send you some timber: but your Blessedness has not indicated whether you are in need of it: and we can send some of much larger size, but no ship is sent hither capable of containing it: and I think shame to send the smaller sort. Nevertheless let your Blessedness inform me by letter what I should do. I have however sent you, as a small blessing from the Church of Saint Peter who loves you, six of the smaller sort of Aquitanian cloaks (pallia), and two napkins (oraria); for, my affection being great, I presume on the acceptableness of even little things. For affection itself has its own worth, and it is quite certain that there will be no offence in what out of love one has presumed to do. Moreover I have received the blessing of the holy Evangelist Mark, according to the note appended to your letter. But, since I do not drink colatum and viritheum with pleasure, I venture to ask for cognidium, which last year, after a long interval, your Holiness caused to be known in this city. For we here get from the traders the name of cognidium, but not the thing itself. Now I beg that the prayers of your Holiness may support me against all the bitternesses which I suffer in this life, and defend me from them by your intercessions with Almighty God. http://www.clerus.org/clerus/dati/2001-02/13-999999/e7.html