43 Now, regarding that lengthened sermon which, according to Matthew, the Lord delivered on the mount, let us at present see whether it appears that the rest of the evangelists stand in no manner of antagonism to it. Mark, it is true, has not recorded it at all, neither has he preserved any utterances of Christ's in any way resembling it, with the exception of certain sentences which are not given connectedly, but occur here and there, and which the Lord repeated in other places.
Nevertheless, he has left a space in the text of his narrative indicating the point at which we may understand this sermon to have been spoken, although it has been left unrecited. That is the place where he says: “And He was preaching in their synagogues, and in all Galilee, and was casting out devils.” Under the head of this preaching, in which he says Jesus engaged in all Galilee, we may also understand that discourse to be comprehended which was delivered on the mount, and which is detailed by Matthew.
For the same Mark continues his account thus: “And there came a leper to Him, beseeching Him; and kneeling down to Him, said, If You will, You can make me clean.” And he goes on with the rest of the story of the cleansing of this leper, in such a manner as to make it intelligible to us that the person in question is the very man who is mentioned by Matthew as having been healed at the time when the Lord came down from the mount after the delivery of His discourse. For this is how Matthew gives the history there: “Now, when He had come down from the mountain, great multitudes followed Him; and, behold, there came a leper, and worshipped Him, saying, Lord, if You will, You can make me clean;” and so on.
Source: The Harmony of the Gospels (New Advent)