Musnad Ahmad 42:3

Abdullah ibn Ahmad said: I found in my father’s book, in his own handwriting: Ya’la ibn ‘Ubayd narrated to us, Sufyan narrated to us, from Mansur, from Mujahid, from al-Hakam ibn Sufyan—or Sufyan ibn al-Hakam—who said: "I saw the Messenger of Allah (peace and blessings of Allah be upon him) urinate, then—meaning—he sprinkled water on his private part."