Genesis 21:27-31

So Abraham took sheep and oxen and gave them to Abimelech, and the two men made a covenant (binding agreement). Then Abraham set apart seven ewe lambs of the flock, and Abimelech said to Abraham, “What is the meaning of these seven ewe lambs which you have set apart?” Abraham said, “You are to accept these seven ewe lambs from me as a witness for me, that I dug this well.” Therefore that place was called Beersheba (Well of the Oath or Well of the Seven), because there the two of them swore an oath.