Mishkat al-Masabih #342

‘A'isha told of God’s messenger passing water and ‘Umar 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 pass water. If I were to do so it would become a sunna” Abu Dawud and Ibn Majah transmitted it.