Sahih al-Bukhari 224
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · To pass urine while standing and sitting
Narrated Hudhaifa:
حَدَّثَنَا آدَمُ ، قَالَ حَدَّثَنَا شُعْبَةُ ، عَنِ الأَعْمَشِ ، عَنْ أَبِي وَائِلٍ ، عَنْ حُذَيْفَةَ ، قَالَ أَتَى النَّبِيُّ صلى الله عليه وسلم سُبَاطَةَ قَوْمٍ فَبَالَ قَائِمًا، ثُمَّ دَعَا بِمَاءٍ، فَجِئْتُهُ بِمَاءٍ فَتَوَضَّأَ.
Once the Prophet (ﷺ) went to the dumps of some people and passed urine while standing. He then asked for water and so I brought it to him and he performed ablution.