Genesis 21:27-31

So Abraham took sheep and cattle, gave them to Abimelech, and the two men made a covenant. Then Abraham set apart from his flock seven ewe lambs, and Abimelech asked him, “What are you doing with these seven ewe lambs you have set apart?” He replied, “I am giving them to you, and by accepting these seven ewe lambs, you acknowledge the proof that I dug this well and it belongs to me.” That place was called Beersheba, because it was there that both men swore an oath to one another.