Genesis 21:31-33

Therefore he called that place Beersheba, because the two of them swore an oath there. Thus they made a covenant at Beersheba. So Abimelech rose with Phichol, the commander of his army, and they returned to the land of the Philistines. Then Abraham planted a tamarisk tree in Beersheba, and there called on the name of the LORD, the Everlasting God.