פירוש על במדבר 29:33

That is that He commanded to offer an additional sacrifice on the eighth day of the Holiday (Sukkot). And that is the additional offering of the eighth day holiday of Atzeret. And that which obligates us to count this additional offering separately - separate from all the days of Sukkot - it that we expound that the eighth day Atzeret is its own festival. And that is completely clear. (See Parashat Pinchas; Mishneh Torah, Daily Offerings and Additional Offerings 10.)