4 This is the first Commandment,— You shall love the Lord your God with all your heart and your soul, and with your might according as you are able. The sign that you love God, is this, that you love your fellow; and if you hate your fellow, your hatred is towards God. For it is blasphemy if you pray before God while you are angry. For your heart also convicts you, that in vain you multiply words: your conscience rightly judges that in your prayers you profit nought. Christ as He hung on the height of the tree, interceded for His murderers; and you (who art) dust, son of the clay, rage fills you at its will. You keep anger against your brother; and do you yet dare to pray? Even he that stands on your side, though he be not neighbour to your sins, the taint of iniquity reaches unto him, and his petition is not heard. Leave off rage and then pray; and unless you would further provoke, restrain anger and so shall you supplicate. And if he (the other) is not to encounter you in fury, banish rage from that body, because it is holden with lusts.
Source: On Admonition and Repentance (New Advent)