14 What then, my brethren, what is said of our Head? “For Thou, Lord, art my hope,” etc. Of this we have spoken, “for He has given His angels charge over You, to keep You in all Your ways”. You heard these words but now, when the Gospel was being read; attend therefore. Our Lord, after He was baptized, fasted. Why was He baptized? That we might not scorn to be baptized. For when John said to our Lord, “Comest Thou to me to be baptized? I ought to be baptized by You;” and our Lord replied, “Suffer it to be so now, for thus it becomes us to fulfil all righteousness;” He wished to fulfil all humility, so that He should be washed, who had no defilement....Our Lord, then, was baptized, and after baptism He was tempted; He fasted forty days, a number which has, as I have often mentioned, a deep meaning.
All things cannot be explained at once, lest needful time be too much taken up. After forty days He was an hungred. He could have fasted without ever feeling hunger; but then how could He be tempted? Or had He not overcome the tempter, how couldest thou learn to struggle with him? He was hungry; and then the tempter said, “If Thou be the Son of God, command that these stones be made bread.” Was it a great thing for our Lord Jesus Christ to make bread out of stones, when He satisfied so many thousands with five loaves?
He made bread out of nothing. For whence came that quantity of food, which could satisfy so many thousands? The sources of that bread are in the Lord's hands. This is nothing wonderful; for He Himself made out of five loaves bread enough for so many thousands, who also every day out of a few seeds raises up on earth immense harvests. These are the miracles of our Lord: but from their constant operation they are disregarded. What then, my brethren, was it impossible for the Lord to create bread out of stones?
He made men even out of stones, in the words of John the Baptist himself, “God is able of these stones to raise up children unto Abraham.” Why then did He not so? That he might teach you how to answer the tempter, so that if you were reduced to any straits and the tempter suggested, if you were a Christian and belonged to Christ, would He desert you now?...Listen to our Lord: “Man shall not live by bread alone, but by every word that proceeds out of the mouth of God.” Do you think the word of God bread? If the Word of God, through which all things were made, was not bread, He would not say, “I am the bread which came down from heaven.” You have therefore learned to answer the tempter, when pressed with hunger.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)