Musnad Abu Ya'la al-Mawsili 3:196

Abu Khaythama narrated to us, Ibn 'Uyaynah narrated to us, he heard 'Amr [narrate] from 'Ata' from 'A'ish ibn Anas, [who] heard 'Ali narrating to the people upon the pulpit: I said to 'Ammar, "Ask the Messenger of Allah (may Allah bless him and grant him peace) for me about pre-seminal fluid (madhy), for his daughter is under my care, and I am too shy to ask him." So he asked him, and he said, "When he finds that, let him perform ablution."