Genesis 23:19

And afterwards Abraham buried Sarah his wife in the cave of the field Kapheilta which is before Mamre, that is, Hebron in the land of Kenaan.

AND AFTER THIS, ABRAHAM BURIED SARAH: After [the burial of Sarah] the field was made sure as a burial ground possession belonging to Abraham and his descendents. Torah tells [of the purchase of the field of Machpelah] ...it informs us that G-d's word to Abraham that he would possess the land as an inheritance was fullfilled.

AND AFTER THIS, ABRAHAM BURIED SARAH HIS WIFE IN THE CAVE OF THE FIELD OF MACHPELAH BEFORE MAMRE — THE SAME IS HEBRON — IN THE LAND OF CANAAN. The reason why Scripture reverts to clarify the field, the place and the land is that the whole section mentioned the sons of Heth and Ephron the Hittite. Therefore Scripture mentions at the conclusion that the field was in the land of Canaan which is the land of Israel. And so it said at the beginning of the section: in Kiryath arba — the same is Hebron — in the land of Canaan. All this is to explain that the righteous woman died in the land of Israel, and there she was interred, as the Hittites were of the families of Canaan., 10:15.
In my opinion, the reason for the verses is only to mention that this was the land of Canaan, not the land of the Philistines. Having said, And Abraham sojourned in the land of the Philistines many days,, 21:34. and since all his habitats were in that land — in Gerar,, 20:1. and the valley of Gerar, and Beer-sheba, and from there to Hebron and back — Scripture therefore mentioned that Hebron is in the land of the Canaanite who dwelt in that hill country, not in the land of the Philistines, which is mentioned in connection with Abraham. And in the end Scripture added a phrase to inform us that the cave was in the field of Machpelah before Mamre, for this was the name by which it was known.
This section was written to inform us of G-d’s kindnesses to Abraham, i.e., that in the land in which he came to live he was regarded as a prince of G-d, and each individual as well as all the people called him “my lord” although he did not tell them that he was a prince and a great man. Also, in his lifetime G-d fulfilled His promise to him: And I will make thy name great, and thou shalt be a blessing., 12:2. Moreover, his wife died and was buried in the inheritance of the Eternal. Further, Scripture wanted to inform us of the place of the burial of the patriarchs since we are obligated to honor the burial place of our holy ancestors. Our Rabbis said that this also was one of the trials of Abraham: he desired a place to bury Sarah but did not find it until he purchased it, [despite G-d’s promise that the entire land would be given to him].
I do not know a reason for the words of Rabbi Abraham ibn Ezra, who says that the purpose of this section is to let us know the superiority of the land of Israel as regards the living and the dead, and also to fulfill the word of G-d which promised him that the land would be an inheritance of his. But what superiority of the land was thus demonstrated? Abraham would not have carried her to another land to bury her, and the word of G-d to Abraham applied to the whole land, and that was fulfilled only with his seed.

ואחרי כן...אל מערת, the preposition אל means the same as the prefix ב, i.e. במערת שדה המכפלה. Compare Exodus 25,21 ואל הארון תתן את העדות instead of בארון.