Musnad Ahmad 559:2

Yahya ibn Sa'id narrated to us, from Sufyan, who said: Mansur narrated to me. And 'Abd al-Rahman ibn Mahdi said: Sufyan and Za'idah narrated to us, from Mansur, from Mujahid, from al-Hakam ibn Sufyan—or Sufyan ibn al-Hakam. 'Abd al-Rahman said in his narration: "I saw the Messenger of Allah (may Allah bless him and grant him peace) urinate, then he performed ablution and sprinkled water on his private part."