سنن ابن ماجه

It was narrated that 'Aishah said: "Mention was made in the presence of the Messenger of Allah (ﷺ) of some people who did not like to face towards the Qiblah with their private parts. He said: 'I think that they do that. Turn my seat (in the toilet) to face the Qiblah.'" (Da'if) Another chain with similar wording.