We read in The Spiritual Meadow [of John Moschos, Chapter 147]:
“Abba Menas, superior of Salams, a coenobium near Alexandria, related that he heard this from Eulogius, Patriarch of Alexandria: ‘While staying in Constantinople, I was a guest in the house of my lord Gregory, archdeacon of the Church of Rome, [Saint Gregory the Dialogist, later Pope of Rome] a truly illustrious and virtuous. He told me a story recorded in the archives of the Roman Church about the most blessed and Most Holy Pope Leo. He said that Leo wrote a letter to Saint Flavian, Bishop of Constantinople, condemning the impious Euthyches and Nestorius, and put it on the tomb of Peter, the chief Apostle, “if I, as a man, have in this letter erred in any way or failed to explain the truth fully, do thou, to whom this Church and episcopal throne were entrusted, set it right.” Forty days later the Apostle appeared while Leo was praying. He said, “I have read your letter and corrected it.” The Pope took the epistle from the blessed Peter’s tomb, opened it, and found that it had been amended by the Apostle’s hand.’”
John Moschos, the author of the Spiritual Meadow, relates, that he heard Amos, patriarch of Jerusalem, say to the abbots: “Pray for me. The dreadful weight of the priesthood affrights me beyond measure, especially the charge of conferring orders. I have found it written, that the blessed Pope Leo, equal to the angels, watched and prayed forty days at the tomb of St. Peter, begging through the intercession of that apostle to obtain of God the pardon of his sins. After this term, St. Peter, in a vision, said to him: Your sins are forgiven you by God, except those committed by you in conferring holy orders: of these you still remain charged to give a rigorous account.”
St. Leo, with regard to those who are to be ordained ministers of the altar, lays down this rule, inserted in his words into the body of the canon law: “What is it not to lay hands upon any one suddenly, according to the precept of the apostle, but not to raise to the honour of the priesthood any who have not been thoroughly tried, or before a mature age, a competent time of trial, the merit of labour in the service of the church, and sufficient proofs given of their submission to rule, and their love of discipline and zeal for its observance.”