Musnad Ahmad 525:1

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 hadith]: "I saw the Messenger of Allah (may Allah bless him and grant him peace) urinate, then perform ablution, and sprinkle his private part with water." Yahya said in his hadith: "The Prophet (may Allah bless him and grant him peace) urinated and sprinkled [water]."