Matthew 6:16-18
"When you fast, do not look somber as the hypocrites do, they disfigure your faces to show others they are fasting. Truly I tell you, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees that is done in secret, will reward you."