18 so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you.

Read Matthew 6:18 Using Other Translations

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.
that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.
Then no one will notice that you are fasting, except your Father, who knows what you do in private. And your Father, who sees everything, will reward you.