Musnad Abu Ya'la al-Mawsili 150:346

Abu Sa’id al-Qawariri narrated to us, Abu Ya’qub al-Taw’am narrated to us, ‘Abdullah ibn Abi Mulaykah narrated to us, from his father, from ‘Aishah, that the Messenger of Allah (ﷺ) urinated. ‘Umar followed him with a vessel of water. He said, “What is this, O ‘Umar?” He said, “Water for you to perform ablution, O Messenger of Allah.” The Messenger of Allah (ﷺ) said, “I have not been commanded, every time I urinate, to perform ablution. And if I were to do so, it would become a Sunnah.”