Musnad Ahmad 939:16

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Muslim al-Zuhri narrated to me, from 'Urwah ibn al-Zubayr, from Zayd ibn Khalid al-Juhani, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever touches his private part, let him perform ablution."