Musnad Ahmad 1153:2245

'Umar ibn Ayyub al-Mawsili narrated to us, from Ja'far, from al-Zuhri and Kathir, he said: Ja'far narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "He used to see it on the *mirt* (garment) of one of us, then he would rub it off—meaning the fluid—and their *murut* (garments) in those days were of wool." She meant the Prophet, peace and blessings of Allah be upon him.