Matthew 6:16-18

And when ye fast, be not as the hypocrites, downcast in countenance; for they disfigure their faces, so that they may appear fasting to men: verily I say unto you, They have their reward. But thou , when fasting, anoint thy head and wash thy face, so that thou mayest not appear fasting unto men, but to thy Father who is in secret; and thy Father who sees in secret shall render it to thee.