Musnad Ahmad 1153:1594

Yahya ibn Sa'id narrated to us, from al-A'mash, who said: Ibrahim narrated to us, from Hammam ibn al-Harith, from 'Aishah, who said: I used to see the semen on the garment of the Messenger of Allah, may Allah bless him and grant him peace, and I would scrape it off. And Yahya said once: And I would rub it off.