Chrysostom on Mark 9:17-31
The disciples were afraid that they had lost the grace given them by the Lord and had been unable to cast out the demon for this reason. Thus, they approached Him with fear and great reverence, and put a question to Him privately, since they sought to gain understanding of a great and unknown matter. What did the Lord say? This kind, He says, meaning lunacy, or rather every kind of demon, does not depart in any other way than by prayer and fasting. Both the sufferer and the one who would heal ought to fast; fasting is necessary for both, but it is especially required of the sufferer. It behooves him not only to fast, but also to pray, for true prayer is practiced when it has fasting joined with it.

From John Chrysostom, The Gospel Commentary, translated from Slavonic into English by Abbot German Ciuba (Church of the Nativity of Christ, Old Rite, Erie, Pa.)