Genesis 21:27-31

So Abraham took sheep and oxen and gave them to Abimelech, and the two of them made a covenant.

But Abraham set seven ewe lambs of the flock by themselves.

Then Abimelech said to Abraham, “What do these seven ewe lambs mean, which you have set by themselves?”

He said, “You shall take these seven ewe lambs from my hand so that it may be a witness for me, that I dug this well.”

Therefore he called that place Beersheba, because there the two of them took an oath.