Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:14-29

“(ubi sup.) The question, indeed, which was raised may, if I am not deceived, have been this, wherefore they, who were the disciples of the Saviour, were unable to heal the demoniac, who was placed in the midst, which may be gathered from the following words; And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit; and wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away.”