And when you fast, do not put on a sad face as the hypocrites do. They neglect their appearance so that everyone will see that they are fasting. I assure you, they have already been paid in full. When you go without food, wash your face and comb your hair, so that others cannot know that you are fasting — only your Father, who is unseen, will know. And your Father, who sees what you do in private, will reward you.

[Ὅταν δὲ νηστεύητε, μὴ γίνεσθε ὡς οἱ ὑποκριταὶ σκυθρωποί, ἀφανίζουσιν γὰρ τὰ πρόσωπα αὐτῶν ὅπως φανῶσιν τοῖς ἀνθρώποις νηστεύοντες· ἀμὴν λέγω ὑμῖν, ἀπέχουσιν τὸν μισθὸν αὐτῶν. σὺ δὲ νηστεύων ἄλειψαί σου τὴν κεφαλὴν καὶ τὸ πρόσωπόν σου νίψαι, ὅπως μὴ φανῇς τοῖς ἀνθρώποις νηστεύων ἀλλὰ τῷ πατρί σου τῷ ἐν τῷ κρυφαίῳ· καὶ ὁ πατήρ σου ὁ βλέπων ἐν τῷ κρυφαίῳ ἀποδώσει σοι.]

The Bible (The New Testament) (AD 1st - 2nd C) Christian sacred scripture

Matthew 6: 16-18 (Jesus) [GNT (1966)]
    (Source)

No Synoptic parallels.

(Source (Greek)). Alternate translations:

Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. But thou, when thou fastest, anoint thine head, and wash thy face; that thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly. [KJV (1611)]

When you fast do not put on a gloomy look as the hypocrites do: they pull long faces to let men know they are fasting. I tell you solemnly, they have had their reward. But when you fast, put oil on your head and wash your face, so that no one will know you are fasting except your Father who sees all that is done in secret; and your Father who sees all that is done in secret will reward you. [JB (1966)]

When you are fasting, do not put on a gloomy look as the hypocrites do: they go about looking unsightly to let people know they are fasting. In truth I tell you, they have had their reward. But when you fast, put scent on your head and wash your face, so that no one will know you are fasting except your Father who sees all that is done in secret; and your Father who sees all that is done in secret will reward you.
[NJB (1985)]

And when you fast, don’t put on a sad face like the hypocrites. They distort their faces so people will know they are fasting. I assure you that they have their reward. When you fast, brush your hair and wash your face. Then you won’t look like you are fasting to people, but only to your Father who is present in that secret place. Your Father who sees in secret will reward you.
[CEB (2011)]

And whenever you fast, do not look somber, like the hypocrites, for they mark their faces to show others that they are fasting. Truly I tell you, they have received their reward. But when you fast, put oil on your head and wash your face, so that your fasting may be seen not by others but by your Father who is in secret, and your Father who sees in secret will reward you.
[NRSV (2021 ed.)]

The NRSV notes some early manuscripts have the Father rewarding you "openly," which the KJV uses.