Musannaf Ibn Abi Shaybah 597 — Those who, after urinating, did not

Sayyidah Aishah (may Allah be pleased with her) said: Once the Prophet (peace and blessings be upon him) went to urinate, and Sayyiduna Umar (may Allah be pleased with him) followed him with water. The Prophet (peace and blessings be upon him) said: "O Umar! What is this?" He said: "This is water for you to perform ablution with." The Prophet (peace and blessings be upon him) said: "I have not been commanded to perform ablution every time I urinate. If I were to do so, this act would become part of the religion (a sunnah)."