I couldn’t find anywhere in the Sifsei Chachomim that this Rebbi Yitzchok is Rashi’s father. But even if he (or someone else) does say it, with the greatest of respect it isn’t so. In actual fact, this statement of Rebbi Yitzchok appears in the Yalkut Shimoni on the possuk ????? ??? ??? in the name of the Tanchuma; indeed, it can be found in ?????? ??? here in ??????. See also Ramban on this Rashi who calls it an ????, a term he reserves for ????? ??”?.
