'Fasting' in the Bible

"Whenever you fast, do not be sullen like the hypocrites, for they make their faces unrecognizable in order that they may be seen fasting by people. Truly I say to you, they have received their reward in full!

But [when] you are fasting, {put olive oil on your head} and wash your face

so that you will not be seen by people as fasting, but to your Father who is in secret, and your Father who sees in secret will reward you.