44 This leper is also referred to by Luke, not indeed in this order, but after the manner in which the writers are accustomed to act, recording at a subsequent point things which have been omitted at a previous stage, or bringing in at an earlier point occurrences which took place at a later period, according as they had incidents suggested to their minds by the heavenly influence, with which indeed they had become acquainted before, but which they were afterwards prompted to commit to writing as they came up to their recollection.
This same Luke, however, has also left us a version of his own of that copious discourse of the Lord, in a passage which he commences just as the section in Matthew begins. For in the latter the words run thus: “Blessed are the poor in spirit: for theirs is the kingdom of heaven;” while in the former they are put thus: “Blessed be ye poor: for yours is the kingdom of God.” Then, too, much of what follows in Luke's narrative is similar to what we have in the other. And finally, the conclusion given to the sermon is repeated in both Gospels in its entire identity—namely, the story of the wise man who builds upon the rock, and the foolish man who builds upon the sand; the only difference being, that Luke speaks only of the stream beating against the house, and does not mention also the rain and the wind, as they occur in Matthew.
Accordingly, it might very readily be believed that he has there introduced the self-same discourse of the Lord, but that at the same time he has omitted certain sentences which Matthew has inserted; that he has also brought in other sayings which Matthew has not mentioned; and that, in a similar manner, he has expressed certain of these utterances in somewhat different terms, but without detriment to the integrity of the truth.
Source: The Harmony of the Gospels (New Advent)