Matthew 6:16 - 18

16 “Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward.

17 But you, when you fast, anoint your head and wash your face,

18 so that you do not appear to men to be fasting, but to your Father who is in the secret place; and your Father who sees in secret will reward you openly.

Fasting has never been commanded for Christians to practice regularly, but it is a voluntarily action when done as a symbol of grief, or being repentant of sins committed, or in times of being anxious, such as times when making serious decisions.

These are actions which are approved when they are prompted by sincere motives, again it is a matter which is condemned when being done to gain favor of mankind by appearing to be righteous.

The hypocrites who fasted made their fast seem more severe than was actually happening, they did this by using means of making their faces seem distorted to look as if they were suffering greatly from lack of food. This fake means of appearing to fast was what was condemned by Christ.

Again Jesus points out that those who gain honor and glory by seeking man's approval, has already gained all of the reward they will get.

In verse 17 Jesus tells his disciples to do the opposite, do what it takes to appear as if you are not fasting, wash your hair and face to appear as if normal.

Again God sees the secret actions and rewards those who are doing what is right but doing it so as to not make it intentionally public.