Musnad Ahmad 1153:55

Muhammad ibn Abi ‘Adi narrated to us, from Sa‘id, from Abu Ma‘shar, from an-Nakha‘i, from al-Aswad, from ‘Aishah, who said: "I used to rub it off from the garment of the Messenger of Allah, may Allah bless him and grant him peace. If I saw it, I would wash it, and if not, I would sprinkle it."