Matthew 17:21 KJV
Howbeit this kind goeth not out but by prayer and fasting.
The New American Standard Bible puts this verse between brackets and has a footnote saying that early manuscripts do not contain this verse.
Matthew 17:21 NASB
[ But this kind does not go out except by prayer and fasting.” ]
The Revised Version omits this verse, and puts in a footnote: “Many authorities, some ancient, insert verse 21” followed by the text of the verse.
This verse closely resembles Mark 9:29, but it is lacking in Matthew in early manuscripts, and some Italic & Syriac and Coptic and Ethiopic manuscripts.
It is, however, found in this place in some Greek manuscripts which are not quite so ancient, as well as some other manuscripts of the ancient versions.
It is believed to have been assimilated from Mark.