Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 42
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.