7 These expressions, brethren, are safe ones: but yet be watchful in good works. Touch “the psaltery,” by obeying the Commandments; touch the harp, by patiently enduring your sufferings. You have heard from Isaiah, “Break your bread to the hungry;” think not that fasting by itself is sufficient. Fasting chastens your own self: it does not refresh others. Your distress will profit you, if you afford comfort to others. See, you have denied yourself; to whom will you give that of which you have deprived yourself?
Where will you bestow what you have denied yourself? How many poor may be filled by the breakfast we have this day given up? Fast in such a way that you may rejoice, that you have breakfasted, while another has been eating; fast on account of your prayers, that you may be heard in them. For He says in that passage, “Whilst you are yet speaking I will say, Here I am,” provided you will with cheerful mind “break your bread to the hungry.” For generally this is done by men reluctantly and with murmurs, to rid themselves of the wearisome importunity of the beggar, not to refresh the bowels of him that is needy.
But it is “a cheerful giver” that “God loves.” If you give your bread reluctantly, you have lost both the bread, and the merit of the action. Do it then from the heart: that He “who sees in secret,” may say, “while you are yet speaking, Here I am.” How speedily are the prayers of those received, who work righteousness! And this is man's righteousness in this life, fasting, alms, and prayer. Would you have your prayer fly upward to God? Make for it those two wings of alms and fasting. Such may God's “Light” and God's “Truth” find us, that He may find us without cause for fear, when He comes to free us from death, who has already come to undergo death for us. Amen.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)