Luke 6:18

But observe all things carefully, how He both ascends with His Apostles and descends to the multitude; for how could the multitude see Christ but in a lowly place. It follows him not to the lofty places, it ascends not the heights. Lastly, when He descends, He finds the sick, for in the high places there can be no sick.

(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.