Catechism of the Catholic Church §1429 — excerpt
St. Peter's conversion after he had denied his master three times bears witness to this. Jesus' look of infinite mercy drew tears of repentance from Peter and, after the Lord's resurrection, a threefold affirmation of love for him.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Ambrose, ep. 41, 12: PL 16, 1116