Rashi on Genesis 46:1/רש"י על בראשית מ״ו:א׳

1The suffix ה replaces the prefix ל locale (Yevamot 13b).

2לאלהי אביו יצחק TO THE GOD OF HIS FATHER, ISAAC — The duty of honouring one’s father is more imperative than that of honouring one’s grandfather (Genesis Rabbah 94:5); therefore the sacrifices are associated with the name of Isaac and not with that of Abraham.