וַיֵּרָ֨א אֵלָ֤יו יְהוָה֙ בַּלַּ֣יְלָה הַה֔וּא וַיֹּ֕אמֶר אָנֹכִ֕י אֱלֹהֵ֖י אַבְרָהָ֣ם אָבִ֑יךָ אַל־תִּירָא֙ כִּֽי־אִתְּךָ֣ אָנֹ֔כִי וּבֵֽרַכְתִּ֙יךָ֙ וְהִרְבֵּיתִ֣י אֶֽת־זַרְעֲךָ֔ בַּעֲב֖וּר אַבְרָהָ֥ם עַבְדִּֽי׃
און גאָט האָט זיך באַװיזן צו אים אין יענער נאַכט, און האָט געזאָגט: איך בין דער גאָט פֿון דײַן פֿאָטער אבֿרהמען; זאָלסט ניט מוֹרא האָבן, װאָרום איך בין מיט דיר, און איך װעל דיך בענטשן, און װעל מערן דײַן זאָמען, פֿון מײַן קנעכט אבֿרהמס װעגן.
Chizkuni
אל תירא כי אתך אני, “Do not fear for I am with you;” G-d appeared to him at this point as he had been greatly disturbed by the hostile attitude displayed by the Philistines time and again.
Ramban
FEAR NOT, FOR I AM WITH THEE. Since Abimelech drove him away because of the Philistines’ jealousy of him, and since the herdsmen of Gerar quarrelled with him, Isaac feared lest they gather against him and smite him and his family. Therefore the Holy One, blessed be He, assured him that he should not fear them, and He blessed him. Then He prompted them so that the king went to Isaac with greater honor than that which he did to his father since he came with Phichol, the chief of his host, and also brought with him many of his friends.
Sforno
אל תירא, that they would reduce your heritage due to their constant interference with you.