Albert Barnes Commentary Luke 6:17

"and he came down with them, and stood on a level place, and a great multitude of his disciples, and a great number of the people from all Judaea and Jerusalem, and the sea coast of Tyre and Sidon, who came to hear him, and to be healed of their diseases;" — Luke 6:17 (ASV)

And stood in the plain. It is not affirmed, however, that he stood in the plain when he delivered the following discourse. There has been some doubt whether the following discourse is the same as that recorded in the 5th, 6th, and 7th chapters of Matthew, or whether the Saviour repeated the substance of that discourse, and that Luke recorded it as he repeated it. The reasons which have led many to suppose that they refer to the same are:

1. That the beginning and the close are alike.
2. That the substance of each is the same.
3. That after the discourse was delivered, both affirm that Jesus went to Capernaum and healed the servant of the centurion (Matthew 8:5–13; Luke 7:1–10).

On the other hand, Matthew says that the sermon was delivered on the mountain (Matthew 5:1); it is thought to be implied that Luke affirms that it was in the plain.

Matthew says that he sat; Luke, that he stood. Yet there is no reason to suppose that there is a difference in the evangelists.

Jesus spent the night on the mountain in prayer. In the morning he descended into the open plain and healed many. While there, as Luke says, he "stood" and received those who came to him, and healed their diseases.

There is no impropriety in supposing that, being pressed by multitudes, he retired into the mountain again, or to an eminence in the plain, or to the side of the mountain, where the people might be more conveniently arranged and seated to hear him. There he sat, as recorded by Matthew, and delivered the discourse; for it is to be observed that Luke does not say that he delivered the sermon on the plain, but only that he healed the sick there.

Tyre and Sidon. See Barnes on Matthew 11:21.