Musnad Ahmad 1153:2376

Hassan narrated to us, Hammad—meaning Ibn Zayd—narrated to us, Abu Hashim narrated to us, from Abu Mijlaz, from al-Harith ibn Nawfal, from Aisha, who said: "I used to rub the seminal fluid off the garment of the Messenger of Allah, may Allah bless him and grant him peace."