Of the Occasion on Which He Calls Them to Take His Yoke and Burden Upon Them, and of the Question as to the Absence of Any Discrepancy Between Matthew and Luke in the Order of Narration
80 Matthew proceeds thus: “At that time Jesus answered and said, I make my acknowledgment to You, O Father, Lord of heaven and earth, that You have hid these things from the wise and prudent,” and so on, down to where we read, “For my yoke is easy, and my burden is light.” This passage is also noticed by Luke, but only in part. For he does not give us the words, “Come unto me, all you that labour,” and the rest. It is, however, quite legitimate to suppose that all this may have been said on one occasion by the Lord, and yet that Luke has not recorded the whole of what was said on that occasion.
For Matthew's phrase is, that “at that time Jesus answered and said;” by which is meant the time after His upbraiding of the cities. Luke, on the other hand, interposes some matters, although they are not many, after that upbraiding of the cities; and then he subjoins this sentence: “In that hour He rejoiced in the Holy Spirit, and said.” Thus, too, we see that even if Matthew's expression had been, not “at that time,” but “in that very hour,” still what Luke inserts in the interval is so little that it would not appear an unreasonable thing to give it as all spoken in the same hour.
Source: The Harmony of the Gospels (New Advent)