Matthew 6:16-18

Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may be seen of men to fast. Verily I say unto you, They have received their reward. But thou, when thou fastest, anoint thy head, and wash thy face; that thou be not seen of men to fast, but of thy Father which is in secret: and thy Father, which seeth in secret, shall recompense thee.