Genesis 21:27-31

Then Abraham gave Abimelech some sheep and cattle, and they made an agreement. Abraham also put seven female lambs in front of Abimelech. Abimelech asked Abraham, “Why did you put these seven female lambs by themselves?” Abraham answered, “Accept these lambs from me to prove that you believe I dug this well.” So that place was called Beersheba because they made a promise to each other there.