46 But, again, as regards what Matthew proceeds to state after the termination of that discourse— namely this, “And it came to pass, when Jesus had ended these sayings, the people were astonished at His doctrine,” — it may appear that the speakers there were those multitudes of disciples out of whom He had chosen the twelve. Moreover, when the evangelist goes on immediately in these terms, “And when He had come down from the mountain, great multitudes followed Him; and, behold, there came a leper and worshipped Him,” we are at libertyto suppose that that incident took place subsequently to both discourses—not only after the one which Matthew records, but also after the one which Luke inserts.
For it is not made apparent what length of time elapsed after the descent from the mountain. But Matthew's intention was simply to indicate the fact itself, that after that descent there were great multitudes of people with the Lord on the occasion when He cleansed the leper, and not to specify what period of time had intervened. And this supposition may all the more readily be entertained, since [we find that] Luke tells us how the same leper was cleansed at a time when the Lord was now in a certain city—a circumstance which Matthew has not cared to mention.
Source: The Harmony of the Gospels (New Advent)