Musnad Ahmad 1153:642

Muhammad ibn Ja‘far narrated to us, he said: Sa‘id narrated to us, from Abu Ma‘shar, from al-Nakha‘i, from al-Aswad ibn Yazid, from ‘Aishah, that she said: "I used to rub it off the garment of the Messenger of Allah, may Allah bless him and grant him peace, with my hand. If I saw it, I would wash it, and if it was hidden from you, then sprinkle it with water."