Sunan an-Nasai 14772
In-book no 36أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا ابْنُ الْمُبَارَكِ، عَنْ مَعْمَرٍ، عَنِ الأَشْعَثِ بْنِ عَبْدِ اللَّهِ، عَنِ الْحَسَنِ، عَنْ عَبْدِ اللَّهِ بْنِ مُغَفَّلٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ يَبُولَنَّ أَحَدُكُمْ فِي مُسْتَحَمِّهِ فَإِنَّ عَامَّةَ الْوَسْوَاسِ مِنْهُ " .
It was narrated from 'Abdullah bin Mughaffal that the Prophet (ﷺ) said: "None of you should urinate in the place where he bathes, for most Waswas (devilish whispers) come from that." I.e., with regard to whether the urine has soiled his body or not.