It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) urinated, and Sayyiduna Umar (may Allah be pleased with him) came with a cup of water. He (peace and blessings be upon him) said: "O Umar, what is this?" Sayyiduna Umar (may Allah be pleased with him) said: You may perform ablution with it. He (peace and blessings be upon him) said: "I have not been commanded to perform ablution every time I urinate, and if I were to do so, it would become a Sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 551