Genesis 21:31-33

Therefore that place was called Beersheba (Well of the Oath or Well of the Seven), because there the two of them swore an oath. So they made a covenant at Beersheba; then Abimelech and Phicol, the commander of his army, got up and returned to the land of the Philistines. Abraham planted a tamarisk tree at Beersheba, and there he called on the name of the LORD [in prayer], the Eternal God.