Genesis 21:31 in the World English Bible
Therefore he called that place Beersheba, because they both swore an oath there.
Context
30 And he said, For these seven ewe lambs shalt thou take of my hand, that they may be a witness unto me, that I have digged this well.
31 Wherefore he called that place Beer-sheba; because there they sware both of them.
32 Thus they made a covenant at Beer-sheba: then Abimelech rose up, and Phichol the chief captain of his host, and they returned into the land of the Philistines.