Genesis 21:30

And he said
That is, Abraham replied to Abimelech: for these seven ewe lambs shall thou take of my hand;
as a present from him, to be retained as his own: that they may be a witness to me that I have digged this well:
these were to be a testimony that the well that had been taken away from Abraham was one that he had dug, and was his property, and which Abimelech acknowledged by his acceptance of these seven lambs; and very probably Abraham received a note from the hand of Abimelech, owning his reception of the seven lambs, and his title to the well, which these were a witness of.