That's the rule:
And when you fast, do not take on a melancholic air like the hypocrites, who disfigure their faces to show men that they fast. Verily, I say unto you, they have already received their reward.
17 But when you fast, smell your head and wash your face, 18 that people may not see you fasting, but only your Father who is in secret; and your Father who sees in secret will reward you.
Fasting, when it is done for a time, must be done in secret!