Musnad Ahmad 5:616

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard Sulayman narrating from al-Mundhir al-Thawri, from Muhammad ibn ‘Ali, from ‘Ali, he said: I felt shy to ask the Prophet, may Allah bless him and grant him peace, about prostatic fluid (madhy) because of Fatimah, so I ordered al-Miqdad ibn al-Aswad, and he asked the Prophet, may Allah bless him and grant him peace, about that. He said: "In it is wudu (ablution)."