Sunan Ibn Majah
327
Daif+3

It was narrated that 'Aishah said:"The Prophet went out to urinate, and 'Umar followed him with water. He said: 'What is this, O 'Umar?' He said: 'Water.' He said: 'I have not been commanded to perform ablution every time I urinate. If I did that it would have become a Sunnah

DaifAl-Albani
DaifMuhammad Fouad Abd al-Baqi