Rabanus Maurus
Catena Aurea · Matthäusevangelium 17
Then came the disciples to Jesus apart, and said, Why could not we cast him out? And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting.
But while He teaches the Apostles how the dæmon ought to be cast out, He instructs all in regulation of life; that we may all know that all the heavier inflictions, whether of unclean spirits, or temptations of men, may be removed by fasts and prayers; and that the wrath also of the Lord may be appeased by this remedy alone; whence he adds, Howbeit this kind is not cast out but by prayer and fasting.