Q: The Lord said, But you, when you fast, anoint your head and wash your face, so that your fasting does not appear to men (Matt. 6: 17). Now what shall one do who wishes to fast for some cause pleasing to God, as the saints are often found to have done, when, against his will, it is apparent that he is doing so?
R: This precept refers to those who are engaged in performing the commandment of God in order to be seen by human beings, that they may cure this passion of courting human favour. But when the commandment of the Lord is done for God’s glory, it is naturally unfitting that it be hidden from the lovers of God. The Lord showed this when he said: A city set on a mountain cannot be hidden. Neither do they light a lamp and set it under a bushel and so on (Matt. 5: 14-15).
From the Asketikon of St. Basil the Great
Sancrucensis 