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

“(ubi sup.) Further, our Lord, while teaching the Apostles how the worst devil is to be expelled, gives all of us rules for our life; that is, He would have us know that all the more grievous attacks of evil spirits or of men are to be overcome by fastings and prayers; and again, that the anger of the Lord, when it is kindled for vengeance on our crimes, can be appeased by this remedy alone. But fasting in general is not only abstinence from food, but also from all carnal delights, yea, from all vicious passions. In like manner prayer taken generally, consists not only in the words by which we call upon the Divine mercy, but also in all those things which we do with the devotedness of faith in obedience to our Maker, as the Apostle testifies, when he says, Pray without ceasing. (1 Thess. 5:17)”