It was narrated that 'Umar said:
"The Messenger of Allah saw me urinating while standing, and he said: 'O 'Umar, do not urinate standing up.' So I never urinated whilst standing after that." (IT)
English reference (IT): Vol. 1, Book 1, Hadith 308 (IT)
Arabic reference (IT): Book 1, Hadith 328 (IT)