Matthew 6:16-18

And when you fast, do not choose to become gloomy, like the hypocrites. For they alter their faces, so that their fasting may be apparent to men. Amen I say to you, that they have received their reward. But as for you, when you fast, anoint your head and wash your face, so that your fasting will not be apparent to men, but to your Father, who is in secret. And your Father, who sees in secret, will repay you.