Musnad Ahmad 1153:626

'Affan narrated to us, he said: 'Abdullah ibn Yahya al-Dabbi narrated to me, he said: 'Abdullah ibn Abi Mulaykah narrated to me, from his mother, from 'Aishah, that the Messenger of Allah (may Allah bless him and grant him peace) urinated. 'Umar stood behind him with a pitcher. He (the Prophet) said: "What is this, O 'Umar?" He said: "Water for you to perform ablution with, O Messenger of Allah." He said: "I have not been commanded, every time I urinate, to perform ablution; and if I were to do that, it would be a Sunnah."