Musannaf Ibn Abi Shayba 1:592

Abu Usama narrated to us, from Abdullah bin Yahya al-Taw'am, from Ibn Abi Mulayka, from his mother, from Aisha, who said: The Prophet, peace and blessings of Allah be upon him, went out to urinate, and Umar followed him with water. He (the Prophet) said: "What is this, O Umar?" He said: "Water for you to perform ablution with." He said: "I have not been commanded, every time I urinate, to perform ablution. If I were to do so, it would become a Sunnah."