Sforno on Genesis 26:1/ספורנו על בראשית כ״ו:א׳

1אשר היה בימי אברהם, ever since that time there had not been as severe a famine in the Land Of Canaan as there was at the time the Torah speaks of here, a famine severe enough to cause the inhabitants of the land to migrate in search of food.

2וילך יצחק אל אבימלך, in order to obtain a permit to stay in his country. This teaches us that Yitzchok possessed good manners, seeing he could have claimed this as a right under the treaty between Avraham and Avimelech.