Sayyiduna Urwah bin al-Zubayr (may Allah have mercy on him) said: I went to Marwan bin al-Hakam (may Allah have mercy on him), and we asked: What makes ablution obligatory? Marwan said: Touching the private part. Urwah said: I do not know of this. Marwan said: Busrah bint Safwan (may Allah be pleased with her) informed me that she heard the Messenger of Allah (peace and blessings be upon him) say: "When anyone touches his private part, let him perform ablution." Sahih; recorded by Malik [89].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 616