Charles Ellicott Commentary Mark 9:29

"And he said unto them, This kind can come out by nothing, save by prayer." — Mark 9:29 (ASV)

But by prayer and fasting.—The better manuscripts omit the last two words. It is possible that they may have been added, like the tears of Mark 9:24, to strengthen the words actually spoken, by bringing in what had been found to bring with it a new intensity of spiritual volition, and therefore of power to rescue human souls from the frenzy and despair into which they had been plunged by the unclean spirits that possessed them.

A similar addition of “fasting” to prayer, apparently from a similar ascetic tendency, is found in 1 Corinthians 7:5 (see the Note). In Matthew, both words are found, but some of the most ancient manuscripts omit the whole verse. On the whole, however, there is a balance of evidence in their favour; and, as shown in the Note on Matthew 17:21, what they teach is in harmony with other portions of the teaching both of our Lord and His Apostles.