The Pirke De-Rabbi Eliezer told how Isaac made a covenant with the Philistines, when he sojourned there. Isaac noticed that the Philistines turned their faces away from him. So Isaac left them in peace, and Abimelech and his magnates came after him. Paraphrasing the Pirke De-Rabbi Eliezer told that Isaac asked them why they came to him, after having turned aside their faces from him. Paraphrasing the Pirke De-Rabbi Eliezer told that the Philistines replied that they knew that God would give the Land of Israel to Isaac's descendants, and thus they asked him to make a covenant with them that his descendants would not take possession of the land of the Philistines. And so Isaac made a covenant with them. He cut off one cubit of the bridle of the donkey upon which he was riding, and he gave it to them so that they might thereby have a sign of the covenant.