Catechism of the Catholic Church
1434 - The interior penance of the Christian can be expressed in many and various ways. Scripture and the Fathers insist above all on three forms, fasting, prayer, and almsgiving, which express conversion in relation to oneself, to God, and to others. Alongside the radical purification brought about by Baptism or martyrdom they cite as means of obtaining forgiveness of sins: effort at reconciliation with one's neighbor, tears of repentance, concern for the salvation of one's neighbor, the intercession of the saints, and the practice of charity "which covers a multitude of sins."
Gospel of Matthew - Chapter 6
2 “Thus, when you give to the needy, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by others. Truly, I say to you, they have received their reward. 3 But when you give to the needy, do not let your left hand know what your right hand is doing, 4 so that your giving may be in secret. And your Father who sees in secret will reward you.
5 “And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward. 6 But when you pray, go into your room and shut the door and pray to your Father who is in secret. And your Father who sees in secret will reward you.
16 “And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. 17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.