36 In this paragraph, moreover, which we are at present considering, the same Matthew follows up his account of the calling of those disciples to whom, when they were engaged in fishing, He gave the command to follow Him, by a narrative to the effect that He went about Galilee, teaching in the synagogues, and preaching the gospel, and healing all manner of sickness; and that when multitudes had gathered about Him, He went up into a mountain, and delivered that lengthened sermon [already alluded to]. Thus the evangelist gives us ground for understanding that those incidents which are recorded by Mark after the election of those same disciples, took place at the period when He was going about Galilee, and teaching in their synagogues. We are at liberty also to suppose that what happened to Peter's mother-in-law came in at this point; and that he has mentioned at a later stage what he has passed over here, although he has not indeed brought up at that later point, for direct recital, everything else which is omitted at the earlier.
Source: The Harmony of the Gospels (New Advent)