Sunan an-Nasa'i — The Book of Purification — Hadith No: 36
Translation
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) [1] come from that." [1] I.e., with regard to whether the urine has soiled his body or not