A.T. Robertson Commentary Matthew 6:18

"that thou be not seen of men to fast, but of thy Father who is in secret: and thy Father, who seeth in secret, shall recompense thee." — Matthew 6:18 (ASV)

In secret (εν τω κρυφαιω). Here as in 6:4,6 the Textus Receptus adds εν τω φανερω (openly), but it is not genuine. The word κρυφαιος is here alone in the New Testament, but occurs four times in the Septuagint.