Jesus' teaching on the principle of how to fast.
Matthew 6:16-18 (NIV)
16"When you fast, do not look somber as the hypocrites do, for they
disfigure their faces to show men they are fasting. I tell you the
truth, they have received their reward in full. 17But when you
fast, put oil on your head and wash your face, 18so that it will
not be obvious to men that you are fasting, but only to your
Father, who is unseen; and your Father, who sees what is done in
secret, will reward you.
Please note, Only those who are willing need to do this, its not
a for show kind of thing.
God bless. 
