Genesis 21:31-34

So that place was called Beersheba because they made a promise to each other there. After Abraham and Abimelech made the agreement at Beersheba, Abimelech and Phicol, the commander of his army, went back to the land of the Philistines. Abraham planted a tamarisk tree at Beersheba and prayed to the LORD, the God who lives forever. And Abraham lived as a stranger in the land of the Philistines for a long time.