Musnad Ahmad 1153:2167

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Talha ibn Shujah narrated to us, he said: Warqa’ bint Haram al-Huna’iyyah narrated to me, she said: I heard ‘Aishah saying: "I would often see the janabah (semen) on the garment of the Prophet, may Allah bless him and grant him peace, and I would rub it off."