Adam Clarke on Genesis 26:33

He called it Shebah - This was probably the same well which was called Beersheba in the time of Abraham, which the Philistines had filled up, and which the servants of Isaac had reopened. The same name is therefore given to it which it had before, with the addition of the emphatic letter ה he, by which its signification became extended, so that now it signified not merely an oath or full, but satisfaction and abundance.

The name of the city is Beer-sheba - This name was given to it a hundred years before this time; but as the well from which it had this name originally was closed up by the Philistines, probably the name of the place was abolished with the well; when therefore Isaac reopened the well, he restored the ancient name of the place.