Musnad Ahmad 1039:1

Yahya ibn Sa'id narrated to us, Sufyan narrated to us, and 'Abd al-Rahman ibn Mahdi narrated to us, Sufyan and Za'idah informed 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 and perform ablution, and he sprinkled water on his private part." Yahya said in his narration: "The Prophet (may Allah bless him and grant him peace) urinated and sprinkled water on his private part."