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." (RU)
English reference (RU): Vol. 1, Book 1, Hadith 308 (RU)
Arabic reference (RU): Book 1, Hadith 328 (RU)