פירוש על עבודה זרה 71:6

Here we see the source of the notion that Jews should not eat certain foods made by non-Jews. Daniel refused to eat the king’s bread or drink his wine. The rabbis read from this that he did not drink their oil either. After all, oil is like wine and bread—it is a product refined from raw ingredients. The dispute between Rav and Shmuel is over whether this was a decree for all of Israel. Rav holds that it was, and therefore it can never be overturned. Shmuel holds that this was a private behavior, not obligatory for all of Israel, forever.