St Matthew 6:16-18

And when you fast, be not as the hypocrites, sad. For they disfigure their faces, that they may appear unto men to fast. Amen I say to you, they have received their reward. But thou, when thou fastest anoint thy head, and wash thy face; That thou appear not to men to fast, but to thy Father who is in secret: and thy Father who seeth in secret, will repay thee.