أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا شَرِيكٌ، عَنِ الْمِقْدَامِ بْنِ شُرَيْحٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ مَنْ حَدَّثَكُمْ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم بَالَ قَائِمًا فَلاَ تُصَدِّقُوهُ مَا كَانَ يَبُولُ إِلاَّ جَالِسًا ‏.‏

It was narrated that 'Aishah said: "Whoever tells you that the Messenger of Allah (PBUH) urinated standing up, do not believe him, for he would not urinate except while squatting." Translation Not Available

Book Ref: Sunan An Nisai Book 1 Hadith 29
Web Ref:  Sunan An Nisai Vol 1 Book 1 Hadith 29

---