From a text known title “The Shepherd of Hermas”.  One of the earliest Christian texts we have after the New Testament, dating from the first or second century.  It was very popular among Christians in the second and third centuries.

“This fasting,” says he, “if the commandments of the Lord are kept, is very good. This then is the way that you shall keep this fast which you are about to observe:

First of all, keep yourself from every evil word and every evil desire, and purify your heart from all the empty things of this world. If you keep these things, this fast shall be fully complete for you.

And thus you will do: Having fulfilled what is written, on that day on which you fast you shall taste nothing but bread and water; and from your meats, which you would have eaten, you shall add up the amount of that day’s expenditure, which you would have incurred, and you shall give it to a widow, or an orphan, or to one in want, and so you shall humble your soul, that he that has received from your humiliation may satisfy his own soul, and may pray for you to the Lord.”  (Shepherd 56.5–7)