Genesis 21:28

Abraham complained to Abimelech because of a water well, which Abimelech’s servants had violently taken away.

Abimelech said, “I don’t know who has done this thing. You didn’t tell me, and I didn’t hear of it until today.”

Abraham took sheep and cattle, and gave them to Abimelech. Those two made a covenant.

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

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

Therefore he called that place Beersheba, because they both swore an oath there.