Purification (Kitab Al-Taharah)

Dawud :: Book 1 : Hadith 42

Narrated Aisha, Ummul Mu'minin:

The Prophet (peace_be_upon_him) urinated and Umar was standing behind him with a jug of water. He said: What is this, Umar? He replied: Water for you to perform ablution with. He said: I have not been commanded to perform ablution every time I urinate. If I were to do so, it would become a sunnah.