Charles Ellicott Commentary Genesis 21:28

"And Abraham set seven ewe lambs of the flock by themselves." — Genesis 21:28 (ASV)

Seven ewe lambs. —The word in Hebrew for swearing is a passive verb, literally signifying “to be sevened,” that is, done or confirmed by seven. In this ancient narrative, we see a covenant actually thus made binding. Seven ewe lambs are picked out and placed by themselves, and by accepting these Abimelech bound himself to acknowledge and respect Abraham’s title to the well. Apparently, this manner of ratifying an oath was unknown to the Philistines, as Abimelech asks, “What mean these seven ewe lambs?” However, it is equally possible that this question was dictated by the rules of Eastern courtesy. When Abraham had picked out the lambs, it became Abimelech’s duty to ask the purpose of the act, which was then explained. As soon as the lambs were accepted, the ratification was complete.