On the Posuk "Zochor Es Yom HaShabos L'Kadsho" Rashi says "Tenu Leiv Lizkor Tamid Es Yom HaShabbos She'Im Nizdamen Lecha Chaifetz Yafeh Tehei Mazmino L'Shabbos" (Beitzah 16). (take heart to remember the Shabbos - if you find something nice place it aside for Shabbos.)
Given that we rule according to Bais Hillel, why is Rashi here quoting according to Bais Shamai?