Sforno on Genesis 21:32/ספורנו על בראשית כ״א:ל״ב

1וישובו אל ארץ פלשתים, they returned from Beer Sheva, i.e. proof that Beer Sheva was not part of the land of the Philistines. They had gone there only to speak with Avraham who had gone there to see how his livestock was faring. This is where he tendered the gift to confirm the covenant they had made.