Midrasz do Rodzaju 26:27

For reproof leads to peace. And thus do you find with Abraham, viz. (Bereshith 21:25) "And Abraham reproved Avimelech because of the well, etc." And what is written (of this)? (Ibid. 28) "And the two of them made a covenant." And thus with Israel, viz. (Ibid. 26:27) "And Isaac said to them: Why did you come to me when you hate me and you sent me away from you!" And (Ibid. 31) "And they departed from him in peace."

… And thus (that rebuke leads to peace) do we find with Abraham, viz. (Ibid. 21:28) "And Abraham rebuked Avimelech, etc.", followed by (27) "and the two of them made a covenant." And thus do we find with Isaac, viz. (Ibid. 26:27) "And Isaac said to them: Why have you come to me when you hate me and you sent me away from you?" followed by "… (31) "and Isaac saw them off and they went from him in peace."

Isaac made a covenant with the people of the land, when he sojourned in the land of the Philistines. He noticed that they turned their faces away from him. He went away from them in peace, and Abimelech and all his magnates came after him. He said to them: Ye turned aside your faces from me, and now ye come unto me, as it is said, "And Isaac said unto them, Wherefore are ye come unto me, seeing ye hate me?" (Gen. 26:27). "And they said, We saw plainly that the Lord was with thee" (Gen. 26:28). They said: We know that the Holy One, blessed be He, will give to thy seed in the future all these lands; make a covenant of an oath with us, that thy seed will not take possession of the land of the Philistines. He made a covenant of an oath with them. What did Isaac do? He cut off one cubit of the bridle of the ass upon which he was riding, and he gave it to them that it might be in their hands for a sign of the covenant of the oath.