Sahih al-Bukhari 144
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · While urinating or defecating, never face the Qiblah except when you are screened by a building or a wall or something like that
Narrated Abu Aiyub Al-Ansari:
حَدَّثَنَا آدَمُ ، قَالَ حَدَّثَنَا ابْنُ أَبِي ذِئْبٍ ، قَالَ حَدَّثَنَا الزُّهْرِيُّ ، عَنْ عَطَاءِ بْنِ يَزِيدَ اللَّيْثِيِّ ، عَنْ أَبِي أَيُّوبَ الأَنْصَارِيِّ ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا أَتَى أَحَدُكُمُ الْغَائِطَ فَلاَ يَسْتَقْبِلِ الْقِبْلَةَ وَلاَ يُوَلِّهَا ظَهْرَهُ، شَرِّقُوا أَوْ غَرِّبُوا " .
Allah's Messenger (ﷺ) said, "If anyone of you goes to an open space for answering the call of nature he should neither face nor turn his back towards the Qibla; he should either face the east or the west."