Or HaChaim on Genesis 26:17/אור החיים על בראשית כ״ו:י״ז

1 ויחן בנחל גרר. He camped in the valley of Gerar and dwelled there. Isaac did not want to abandon living in the land of the Philistines since G'd had specifically instructed him to make his residence there (verse 3). Even though the wadi that Isaac now moved to may not have been exactly part of G'd's original commandment to "sojourn in this country etc.," he was, after all, forced by circumstances to leave the capital of Avimelech's country. Perhaps the whole episode may be considered as a test of Isaac's faith in G'd, who, though telling him: "I shall give the land to you and to your descendants," did not allow him to remain for long in that part of the land even as a private citizen. G'd wanted to see if Isaac would question His ways just as He had tested Abraham in a similar fashion.