Genesis 21:27-31

So Abraham brought sheep and cattle and gave them to Abimelek, and the two men made a treaty. Abraham set apart seven ewe lambs from the flock, and Abimelek asked Abraham, “What is the meaning of these seven ewe lambs you have set apart by themselves?” He replied, “Accept these seven lambs from my hand as a witness that I dug this well.” So that place was called Beersheba, because the two men swore an oath there.