Musnad Ahmad 30:1285

Yahya ibn Yazid ibn Abd al-Malik—meaning al-Nawfali—narrated to us, my father mentioned it, from Sa'id ibn Abi Sa'id al-Maqburi, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever touches his private part with his hand, with no covering between them, then ablution (wudu) has become obligatory upon him."