"Whenever you fast, don't be sad-faced like the hypocrites. For they make their faces unattractive so their fasting is obvious to people. I assure you: They've got their reward!
References for Matthew 6:16
But when you fast, put oil on your head, and wash your face,
so that you don't show your fasting to people but to your Father who is in secret. And your Father who sees in secret will reward you.
References for Matthew 6:18