Augustine of Hippo
Catena Aurea · Matthäusevangelium 5
And seeing the multitudes, He went up into a mountain: and when He was set, His disciples came unto Him. And He opened His mouth, and taught them, saying, Blessed are the poor in spirit: for theirs is the kingdom of heaven.
(de Cons. Ev. ii. 19.) It causes a thought how it is that Matthew relates this sermon to have been delivered by the Lord sitting on the mountain; Luke, as He stood in the plain. This diversity in their accounts would lead us to think that the occasions were different. Why should not Christ repeat once more what He said before, or do once more what He had done before? Although another method of reconciling the two may occur to us; namely, that our Lord was first with His disciples alone on some more lofty peak of the mountain when He chose the twelve; that He then descended with them not from the mountain entirely, but from the top to some expanse of level ground in the side, capable of holding a great number of people; that He stood there while the crowd was gathering around Him, and after when He had sate down, then His disciples came near to Him, and so to them and in the presence of the rest of the multitude He spoke the same sermon which Matthew and Luke give, in a different manner, but with equal truth of facts.