Augustine of Hippo
Catena Aurea · Lukasevangelium 6
And why call ye me, Lord, Lord, and do not the things which I say? Whosoever cometh to me, and heareth my sayings, and doeth them, I will shew you to whom he is like: He is like a man which built an house, and digged deep, and laid the foundation on a rock: and when the flood arose, the stream beat vehemently upon that house, and could not shake it: for it was founded upon a rock. But he that heareth, and doeth not, is like a man that without a foundation built an house upon the earth: against which the stream did beat vehemently, and immediately it fell; and the ruin of that house was great.
(de Con. Ev. ii. 19.) Now this long discourse of our Lord, Luke begins in the same way as Matthew; for each says, Blessed are the poor. Then many things which follow in the narration of each are like, and finally the conclusion of the discourse is found to be altogether the same, I mean with respect to the men who build upon the rock and the sand. It might then easily be supposed that Luke has inserted the same discourse of our Lord, and yet has left out some sentences which Matthew has kept, and likewise put in others which Matthew has not; were it not that Matthew says the discourse was spoken by our Lord on the mountain, but Luke on the plain by our Lord standing. It is not however thought likely from this that these two discourses are separated by a long course of time, because both before and after both have related some things like, or the same. It may however have happened that our Lord was at first on a higher part of the mountain with His disciples alone, and that then he descended with them from the mount, that is, from the summit of the mountain to the flat place, that is, to some level ground, which was on the side of the mountain, and was able to hold large multitudes, and that there He stood until the crowds were gathered together to Him, and afterwards when He sat down His disciples came nearer, and to them, and the rest of the multitude who were present, He held the same discourse.