It was narrated that 'Aishah said:
"The Prophet went out to urinate, and 'Umar followed him with water. He said: 'What is this, O 'Umar?' He said: 'Water.' He said: 'I have not been commanded to perform ablution every time I urinate. If I did that it would have become a Sunnah.'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 327 (DE)
Arabic reference (DE): Book 1, Hadith 351 (DE)