وَعَن عمر
قَالَ: رَآنِي النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَأَنَا أَبُولُ قَائِمًا فَقَالَ: «يَا عُمَرُ لَا تَبُلْ قَائِمًا» فَمَا بُلْتُ قَائِمًا بَعْدُ
The Prophet (sallallahu alayhi wa sallam) saw me standing and passing water and said, “Umar, do not pass water standing.” So I never did it again. Tirmidhi and Ibn Majah transmitted it. The shaikh and imam Muhyi as-Sunna has given a sound tradition from Hudhayfah who said that the Prophet (sallallahu alayhi wa sallam) came to a midden and passed water standing. (Bukhari and Muslim.) It is said that that had a valid reason.
Mishkat al-Masabih 339, Book 4, Purification, Hadith 75. 1 min read