Musnad Ahmad 1039:2

Yahya ibn Sa'id narrated to us, Sufyan narrated to us, from Mansur, from Mujahid, from a man from Thaqif—who is al-Hakam ibn Sufyan or Sufyan ibn al-Hakam—who said: "I saw the Messenger of Allah (may Allah bless him and grant him peace) urinate, then he sprinkled water on his private part."