Textual Commentary on Mark 9:28, 29
Mark 9, TEV (Today's English Version Bible, copyright 1976, 1992, American Bible Society. Used by permission. All rights reserved.)
 After Jesus had gone indoors, his disciples asked him privately, "Why couldn't we drive the spirit out?"  Only prayer can drive this kind out," answered Jesus; "nothing else can."
This story was recorded in all three of the synoptic gospels, which include Matthew, Mark, and Luke. (See Mark 9:14-29; Matthew 17:14-21; Luke 9:37-43.) Mark was the earliest of those three gospels. The Matthew and Luke were written ten to twenty years later. So, this ending in Mark is the earliest recorded ending that we have for this story.
The words "and fasting" appeared in some Greek manuscripts, but the textual evidence shows that they were not in the original texts of Matthew, Mark, or Luke. This conclusion is reflected in virtually all modern versions.
The KJV version of Matthew 17 includes verse 21, which has the same wording as Mark 9:29. The textual evidence shows that Matthew 17:21 was not in the original text of Matthew but was simply inserted by a later copiest to make Matthew's account agree with Mark's account of the story.
Copyright 1996-2003 Robert Nguyen Cramer