15 Wherefore also the Lord Jesus, wishing to make us more strong against the temptations of the devil, fasted when about to contend with him, that we might know that we can in no other way overcome the enticements of evil. Further, the devil himself hurled the first dart of his temptations from the quiver of pleasure, saying: “If You be the Son of God, command that these stones become bread.” After which the Lord said: “Man does not live by bread alone, but by every word of God;” and would not do it, although He could, in order to teach us by a salutary precept to attend rather to the pursuit of reading than to pleasure. And since they say that we ought not to fast, let them prove for what cause Christ fasted, unless it were that His fast might be an example to us. Lastly, in His later words He taught us that evil cannot be easily overcome except by our fasting, saying: “This kind of devils is not cast out but by prayer and fasting.”
Source: Letters (New Advent)