Commentary for Genesis 26:24

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.

אל תירא, that they would reduce your heritage due to their constant interference with you.

אל תירא כי אתך אנכי, "do not be afraid for I am with you, etc." Seeing that Isaac had experienced the humiliation of expulsion at the hands of Avimelech and did not know where to turn next, he might have felt abandoned by G'd. This is why G'd reassured him, telling him not to be afraid.