Rabbah told Abbaye and the rest of his students to eat with him on Friday night, lest the light go out at their hosts' houses, in which case they wouldn't have Kiddush there: Pesachim 101a
Rabbah [Rava] bar Rav Nachman's host asked him a halachic question that he couldn't answer; he went to Rava for an answer: Beitzah 12b