MegAntioch 37 :אֵיתוֹ וְנִשבוֹת בִמעָרְתָא דִלמָא נְחַלֵל יוֹמָא דְשַבְתָא‏ : come let us spend sabbath in the cave lest we violate the sabbath day
See the entire entry for šbt vb. to observe the sabbath