What to fast from
Before all else, brothers and sisters, fast from strife and discord. Recall the prophet’s reproach: “On your fast days you follow your own desires, you goad those under your yoke, you beat them with your fists, and is your cry to be heard?” (Is 58:3-5). And to this he added: “This is not the fast I have chosen, says the Lord.”
If you wish to cry out, make use of the cry of which it is written, “With my voice I cried out to the Lord” (Ps 141:2). That’s not a cry of strife but of charity, not of the flesh but of the heart. That’s not the cry of which the prophet spoke: “I looked for him to do judgment and he did iniquity, and to do justice and instead he caused an outcry” (Is 5:7).
“Forgive and you will be forgiven; give and you will be given (Lk 6:37-38). These are the two wings by which prayer flies to God: that you forgive those who have wronged you and that you give to the needy. (Augustine, Sermon 205, 3; PL 38, 1040)