I said to him, “Sir, I do not see the meaning of these similitudes, nor am I able to comprehend them, unless you explain them to me.” “I will explain them all to you,” he said, “and whatever I shall mention in the course of our conversations I will show you. [Keep the commandments of the Lord, and you will be approved, and inscribed among the number of those who observe His commands.] And if you do any good beyond what is commanded by God, you will gain for yourself more abundant glory, and will be more honoured by God than you would otherwise be.
If, therefore, in keeping the commandments of God, you do, in addition, these services, you will have joy if you observe them according to my command.” I said to him, “Sir, whatsoever you enjoin upon me I will observe, for I know that you are with me.” “I will be with you,” he replied, “because you have such a desire for doing good; and I will be with all those,” he added, “who have such a desire. This fasting,” he continued, “is very good, provided the commandments of the Lord be observed.
Thus, then, shall you observe the fasting which you intend to keep. First of all, be on your guard against every evil word, and every evil desire, and purify your heart from all the vanities of this world. If you guard against these things, your fasting will be perfect. And you will do also as follows. Having fulfilled what is written, in the day on which you fast you will taste nothing but bread and water; and having reckoned up the price of the dishes of that day which you intended to have eaten, you will give it to a widow, or an orphan, or to some person in want, and thus you will exhibit humility of mind, so that he who has received benefit from your humility may fill his own soul, and pray for you to the Lord.
If you observe fasting, as I have commanded you, your sacrifice will be acceptable to God, and this fasting will be written down; and the service thus performed is noble, and sacred, and acceptable to the Lord. These things, therefore, shall you thus observe with your children, and all your house, and in observing them you will be blessed; and as many as hear these words and observe them shall be blessed; and whatsoever they ask of the Lord they shall receive.”
Source: The Pastor (or "The Shepherd") (New Advent)