Abu Dawud Book 1, Hadith Number 42.
Chapter : Performing ablution after releiving oneself.
Narated By ‘Aisha, Ummul Mu’minin : The Prophet (PBUH) 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.