How to act when relieving oneself - Section 2 Mishkat al-Masabih 363, 364
Translation
‘Umar said
The Prophet 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 Hudhaifa who said that the Prophet came to a midden and passed water standing. (Bukhari and Muslim.) It is said that that had a valid reason.