Spiritual Reflection for Lent, from St. Peter Chrysologus, Doctor of the Church (5th c.):

"There are three things, my brethren, by which faith stands firm, devotion remains constant, and virtue endures. They are prayer, fasting and mercy. Prayer knocks at the door, fasting obtains, mercy receives.

Prayer, mercy and fasting: these three are one, and they give life to each other. Fasting is the soul of prayer, mercy is the lifeblood of fasting.

Let us use fasting to make up for what we have lost by despising others. If you are to give him yourself, you are never without the means of giving. To make these acceptable, mercy must be added. Fasting bears no fruit unless it is watered by mercy. Fasting dries up when mercy dries up. Mercy is to fasting as rain is to the earth. However you will not be allowed to keep, what you have refused to give to others."

(Saint Peter Chrysologus, Sermon 43; PL 52, 320 e 322)