Rashi on Genesis 21:30/רש"י על בראשית כ״א:ל׳

1בעבור (supply the word זאת) — IN ORDER THAT this (where this, a feminine form in a neuter sense, as is usual in Hebrew, refers to the incident just recorded).

2תהיה לי לעדה MAY BE UNTO ME AS A WITNESS The word עדה means witness in reference to a previous feminine form (in this case to זאת, this, which word Rashi supplied), just as (31:52) “And the pillar is a witness” (עדה the feminine form to correspond with the feminine noun מצבה, whereas the preceding words are עד הגל הזה “this heap is a witness” where עד is masc to agree with Gal.

3כי חפרתי את הבאר THAT I HAVE DIGGED THIS WELL —Abimelech’s herdsmen quarrelled about it, saying, “We have dug it”. They arranged amongst themselves that the well should belong to him who would show himself at the well together with his sheep and at whose approach the waters would rise, and they rose at the approach of Abraham and his sheep (Genesis Rabbah 54:5).